Those "missing benefits" due to the how the SSA re-calculates delayed retirement credits only in January of each year were long a mystery to me. I could never get clarity on this (though I never called the SSA and asked that specific question). I figured that "someone" would know about this on some of SS forums I frequent (Reddit, AARP, Bogleheads, a Facebook group, etc) but clarity never came.
Until last fall in Bogleheads when this came. I can't direct you to one of the POMS (Program Operations Manual System) of the SSA about this particular issue. (The POMS is their "handbook" for how they interpret the Federal laws ("CFR" - Code of Federal Regulations). See the POMS here https://secure.ssa.gov/apps10/)
So some post in Bogleheads last fall related to this issue, of claiming with DRCs at some intermediate time in the year (any month except January). It seems (again, I can't point to a POMS that states this explicitly) that those months of the first year of benefits, which the benefit is based on the DRCs existing at January of that year, will not be "made whole" with a make-up check. The logic is that the benefit is recalculated for any DRCs that were earned during the past year. So the benefit is recalculated for the second year of benefits, and this is based on any additional DRCs earned in the preceding year...those "missing months". But the additional benefit amount for those missing months in the first year of benefits are not paid retroactively.
At the time, last fall, I was toying with the idea of claiming in December or January, or even beyond. With the thought that the "lost" benefit is not much. But after mulling over the discussions on Bogleheads I maintained my plan to claim at 70 (there are strict rules that when claiming at 70 you get the full benefit of all DRCs immediately, no waiting). I recently put in my application for age 70 and just waiting now.
Of course, I could be entirely wrong about those benefits not being paid. Maybe ask the SSA again.