2B vs 2A amounts mismatch

Yes, it is....
both are separate statement.
GSTR 2A is dynamic statement, updated Regularly whereas GSTR 2B is static statement, generated once on 12th of following month.

If someone file late return, say for June month (after 12th July), than that is reflected in 2B of next month (i.e July), but it's reflected in GSTR 2A in month to which it relates(i.e June)...So, it's"2B is greater than amount in my 2A" in July because of last month's credit reflation.

If I take ITC, then what parameters we follow GSTR 2a or 2b