I too am confused with the choice of how slumps are defined so I emailed headquarters and this was their reply on the matter:
"The longest slump is always 1 day greater than the current slump if the cacher is currently in their longest slump.
Essentially the longest slump counts today's date as no cache found already, while current slump does not count today's date."
I guess an analogy could be that, You missed your doctors appointment the day you have one until the minute you show up at his office.
I'm not sure this can be defined as a bug or just just the way it needed to be programmed for one reason or another.