Hi Karin:
The choice among "Period based on date", "Specific period" and "Subledger period" is remembered so that the next Retrieval process uses the same setting used in the most recent Retrieval. It saves the setting separately for each module. For instance, Receivables could remember "Period based on date" while Payables could be "Specific period".
I've never seen the remembered choice change except by the user making a change. The explanation is almost certainly that someone made a change at the end of January, perhaps unaware of what they had clicked on.