I think it's actually interesting - there was a pretty even mix of 10 and 11 chapter volumes until recently. I'm guessing they're going to primarily stick to 10 chapters for the foreseeable future.
61 - 9
62 - 11
63 - 12
64 - 10
65 - 10
66 - 10
67 - 11
68 - 11
69 - 12
70 - 10
71 - 11
72 - 10
73 - 10
74 - 11
75 - 10
76 - 11
77 - 12
78 - 10
79 - 10
80 - 11
81 - 10
82 - 11
83 - 11
84 - 10
85 - 10
86 - 11
87 - 10
88 - 10
89 - 11
90 - 10
91 - 11
92 - 10
93 - 11
94 - 11
95 - 11
96 - 10
97 - 10
98 - 10
99 - 10
100 - 11
101 - 10
Looking at the New World so far:
9 Chapters - 1 Volume
10 Chapters - 21 Volumes
11 Chapters - 16 Volumes
12 Chapters - 3 Volumes
From 95 onward, we have a trend of mostly 10 chapter volumes; 96, 97, 98, and 99; 100 has 11 chapters, then back to 10 for 101.
I wonder if this has to do with Oda's reduced publication rate and new series climbing the sales charts. One Piece sales have declined partially due to putting out 3 volumes per year. If Oda only includes 10 chapters per volume, he's a lot closer to releasing 4 volumes per year. Volume 100 could be a special case as it's the only volume out of the past 6 to include more than 10 chapters. With 10 in 101, we already have 8 chapters yet to be included in a tankoban, meaning Volume 102 is almost finished.
–- Update From New Post Merge ---
@Captain:
That feels like the safest bet, but it still feels a tad off. The last poll only took up 6 pages of space at the end of volume 87. Plus the usual 6ish pages of fanart and 2 - 4 pages of ads for merch and there's still at least six pages left over. Maybe if the full version of the WT100 illustration is done over multiple spreads at the back there…
Either way, there's legitimately never been this much extra space after the last chapter of a volume before so I'm very curious about the reason
Yeah, you're right. It's still strange. I have no idea what they could have included, particularly now that pictures are out.