On a finer grain detail, sMerits are refilled not on a fixed calendar day, but rather exactly 30 days (down to the second) after any given sMerit TX is made.
That's correct. The first time I saw my source merits bag fill up again at the start of the next month, I wondered why my number of available source merits kept going up in the space of a few minutes. While meriting, I usually keep an eye on how much I have available and found myself in a situation where I noticed an increase between two merited posts. We are talking about less than a minute apart. That's when I realized the thing you just mentioned.