Yes, you would get that about maximum number of days when moved from SG to WRAG because CB ESA would only be payable whilst you were in SG.
(That's not actually important as you qualify for IR ESA anyway, but they have to put it).
It does say that they have looked at your award again and reduced it, but that's nothing to do with the number of days, and it doesn't give the real reason for the reduction.
I'd say you have got an oddly worded letter simply because they don't have a 'standard' letter to cover what they have done, so have tried to bodge a different letter.
As you say you have to challenge (MR) why they have changed your group, especially as it's been done on just a telephone assessment.
Ring them and request MR, ask for copies of the ESA85 Assessors report and the LT54 Decision Makers report.