Did you leave the kiln
Did you leave the kiln closed until it got to room temperature? If so that would explain it.
Kilns cool very slowly as they get closer to the outside temperature – much slower than called for in the schedule. Most people (myself included) start venting the kiln when it gets to 500° F or so by wedging the lid open slightly with a little fiber board or similar. Just be careful not to go so fast you thermal shock your work.