Well, one way that makes sense to me is to count the chapters and both prologue and epilogue, if you do that it will result in the sum of 110 chapters (which is exactly what the site shows).
Volume...Number of Chapters (Prologue and Epilogue included)
V1...........12
V2............7
V3............7
V4............5
V4.5.........7
V5............9
V6............7
V7............7
V7.5.........6
V8............9
V9............9
V10..........8
V11..........10
V11.5.......7
TOTAL......110