Why some abbreviations ended with a period, but some not?

I have just bought an Oxford Advanced Learner's Dictionary.

This is it's first page showing some abbreviations used in the dictionary.

My question is: why some abbreviations ended with a period (such as: abbr., adj.), but some don't (such as: C, sb, pt)?

Solution 1:

A close look at how the abbreviations are generated from their full forms should make the answer apparent.

All abbreviations that are created by truncating a (major) part of the word end with a period, while those formed by truncating parts of two words, or two parts, and then combining those, do not end in a dot.

For example, adj. comes from adjective and pl. from plural, but pt is derived from past tense and sth from something.