Hi! I know it's weird for us because we aren't used to it, but the thing is, even the Japanese numbering is off. They released the specials as normal chapters, just following the numbering and leaving a small note. The author also gave each one of them their own spot in the volume index, which is okay; it's weird but fair—her manga, her choice. She is also naming whole main chapters as 'Number part one' and 'Same number part two,' which doesn't help with the numbering either...
Because the BONUS (don't confuse them with the SPECIALS; they are different things) are going to be named ".5", as they aren't chapters (they don't even appear in the volume index, while the specials do appear and were published in the magazine).
That's why we followed the Japanese example and added a small note for people to read and know what is part of the main story and what is not.
This follows the publishing order, respects the individuality of each special as the author intended, is clear for readers, and doesn't affect the story at all.
Thank you for your understanding and support