Hi,
Month End Accrual is based on the Processing class 79 (spec 1 or 2) & Table T51A1 ( HR Postings Accruals. Also we have LDCD (Last date of Document creation.
Based on the above, as you mentioned above, UACGF processes the accruals. There is no hard and fast rule that evey pay period these wgae types to be generated in ACCR table.
This accruals happens when the payment dates and posting dates are not in sync. Pay periods and Posting periods are different.
In you case it must have been the same. Verify them and see the pay periods and posting periods.
When payperiod falls in between two months precisely, this accruals happens. But, the posting pays the same amount which needs to be paid (there won't be any difference in pay)
Also we have T52EL where these wage types are set up as month end accruals or normal posting.
Please refer the below document which gives you a fair understanding.
Regards
Madhav