Why is it listed as 134 chapters?

That's one of the possibility for the Extra Chapters; a side-story the Authors draws for whatever reason during the publication, and replace the issue of a regular Chapter. Generally, they are shorter than the latter, so it may help them recover a little bit, like a week/month with less work.

But there is other possibilities tho, these Extra Chapters may be an old one-shot of the Author, that was included in the Volumes for a few reasons (like pad the Volume a bit, or make the viewers discover how the Author started, ect), or a new Chapter specially drawn for the Volume releases, to thanks the fans.