R's floor() and ceiling() functions are "inaccurate sometimes" or "can cause unexpected results" which is obviously concerning when math is the one thing in my life I thought I could rely on!
Background here and in ?floor:
The realities of computer arithmetic can cause unexpected results, especially with floor and ceiling. For example, we ‘know’ that floor(log(x, base = 8)) for x = 8 is 1, but 0 has been seen on an R platform. It is normally necessary to use a tolerance.
See attached picture: my suspicion is that confusion for used can come from the fact that the RStudio data viewer rounds numbers at a certain depth of decimal places (perhaps 6?) whereas floor() and ceiling() don't round. Possibly this is a niche / low importance thing but until you do enough digging and work out the cause, it can be very confusing. Feasibly this could be a global option, "data viewer rounding decimal places", default 6, delete value for no rounding. In my attached example, this would mean rows 228991 & 2 both showed as 11.99999 instead of 12.00000.