It could be that 1.15b was simply a patch to add on to 1.15a and that it isn't considered the 2nd patch of the month. They might make the 2nd patch of the month a new number, like maybe.... (thinks real hard)... 1.16 or something.
Or... it could be that the entire list of planned revisions and changes and explorations and introspections that Yoshi listed will all be part of 1.15 and changlog will continue through 1.15z or something like that.
Only time will tell though, unless someone has seen something I am missing.
***Have you ever wondered which hurts the most: saying something and wishing you had not, or saying nothing, and wishing you had?***