I have found that for seemingly all texts (across different languages, probably true for all) the word count shown in the reading tab (when tapping the percentage progress in the bottom to see the location) is consistently higher than that shown in the library tab.
Most texts I checked were off by around 25%, so quite significant (although one text was off by less than 1% for whatever reason).
Both pictures below are from the same book, in the same browser on the same device. The example book is in spanish, but I also tested it in russian and portuguese and the same problem persisted.
