Go to www.cambridge.org/core/journals/radiocarbon/issue/A82311735C77097CA82566... ... which takes you to Radiocarbon journal volume 55 Number 4, and scroll down to page 1869-1887 and the article, "Intcal 13 and Marine 13 radiocarbon age calibration curves 0-50,000 years calendar BP" - by Paula Reimer et al. This outlines why Intcal was upgraded in 2013. It seems to involve the addition of marine data and other dating methodologies such as speleotherm sequences and lake varves, as well as sediment cores from lakes and oceans and Pacific corals from a nice research location in the Pacific, Tahiti. It is therefore a general hotchpotch of data and the idea is to tie it all up in one bundle - a classic modelling technique (complicate to obfuscate). The calibration is basically a model. How much faith should be placed in such models as climate models have not proved to be very intuitive of changing weather patterns in the 21st century. They are basically naff. Are calibration models another one of the same breed? Are they as unreliable as climate models?
One issue with radiocarbon and the movement (possibly related to Intcal 13 and earlier versions) is that we now have divergent dates between the Levant, on the one hand, and Egypt and Sumeria/Babylonia on the other. The former dates of 2345/2200BC have become 2600/2500BC (roughly), and these have been adopted in Europe (you only have to look at the revised dates at Stonehenge where the changes in plan of the monument has moved to the new dates). In the Levant we are talking about the end of Early Bronze III and Early Bronze IVA. This coincided with two massive bouts of site destruction, probably as a result of seismic and/or cosmic activity. Now we have a glaring anomaly as in Egypt and what was Sumeria/Babylonia the old dates still stand, leaving us with a bout of two site destruction episodes delineating the end of Early Dynastic III Sumeria and the end of the Akkadian dynasty. Logic would declare there was a connection with the two events - and until the new calibration came into being they were. Indeed, the Akkadians were blamed for the end of EBIII sites in Syria, such as Tell Mardikh (Ebla) as natural catastrophe was not part of mainstream thinking. Therefore armies of conquest were invoked. We had the same thing in the Levant at the end of the Middle Bronze Age. Earthquakes were disliked and the army of Thutmose III inserted instead, opening up an anomaly in the chronology.
The big question is why a difference now exists between the Levant and Egypt/Babylonia. A conspiracy theorist might claim this was due to purposeful manipulation of the data in order to divorce site destructions in the Levant from those in Sumeria/Babylonia. Cynicism would suggest incompetence may have a part to play - but there is another good logical reason why the two events have been torn apart from each other. Politics. Mischievous intent is the most unlikely scenario and it is more likely the new dates solely arise from calibration models. Sites in the Levant, especially in Israel and Jordan, and in Syria until recently, would largely be devoid of political interference - which explains why they are close to the calibrated dates in Europe. However, in what was Babylonia the situation is entirely different. The region has been racked with war for a long time - and archaeologists are thin on the ground. In Egypt there is plenty of evidence of political interference and the director of archaeology was on record as saying he had no faith in science such as radiocarbon. It is unlikely that Intcal 13 has figured in much of the recent research in Egypt. It would certainly not have been employed to the same extent as in Europe and the Levant - which leads to the current comical position. Eventually the whole region will move towards the Intcal 13 model and we will all have to get used to the new calibrated dates (and even more holes in chronology). Moe Mandelkehr's 2300BC Event will have to be relocated a couple of hundred years earlier. It was already guilty of conflating two events into a single episode and therefore already had dating anomalies. However, it has opened up a huge gap in the post Early Bronze period but this may tie in with those archaeologists who would like to date the Thera eruption around the time of the 1628BC low growth ring event (volcanic or cosmic in origin). This means opening up a bigger gap between dynasty 18 and the Iron Age - and a dark age that never diminishes at the end of the LB age.