Sounds like the usual conscription, just spread out instead of at two particular points in time.

[Igel]

Presumably to catch those coming of age for conscription earlier rather than waiting until they're all 6 months or less old.

Unclear that the totals are somehow changing, apart from fluctuations in population of that age.

https://kyivindependent.com/russia-to-draft-year-round-under-proposed-continuous-mobilization/