Why some abbreviations ended with a '.' but some not?

That's a great question—and I think it has no answer. I'd put it down to insufficient forward thinking. I do notice, however, that (except for C and U) it is only the ones whose first few letters appear in sequence are the ones with the periods.

Those are the abbreviations used in that dictionary, not abbreviations used in English, per se. Some of them, such as "sb" and "sth", are not in general use. "C" and "U" are more like symbols. The editor of the dictionary is the only one who could answer your question, though.

There must be some reasons why the editor choose them. And those reasons must not be determined by the editor himself, but by the grammer, or convention. Thank you, all the same.

unimousThere must be some reasons why the editor choose them. And those reasons must not be determined by the editor himself, but by the grammer, or convention. Thank you, all the same.

Yes, but the editor's reasons are his, and you do not need to worry about them. Those abbreviations are for the purposes of that one book, and the editor was free to do it any

enoonAmericans use the period at all times (Dr., Mr.).

Well, some of us don't.

Mister MicawberWell, some of us don't.

What?

Some of us AmEs don't put periods after Mr. Except when they end a sentence.