Every EPUB book contains metadata, including title, author, and subjects (<dc:subject>), etc. Apparently MapleRead is displaying subjects as tags, and lets you completely override them as stated in their online FAQ.
If you unzip your .epub (easier on a computer), you can inspect the (original) subjects/tags in a file, usually in a file called content.opf. Sounds inconvenient? Fortunately, there is a much easier way.
- Go to Book Info view associated with an EPUB book.
- Tap Edit, then Restore All From Book.
- You should the see the subjects/tags as originally embedded in the book. (They may or may not match what you see in calibre, depending on what you've done there.)
- Now tap Cancel, if you don't really want to restore.