Do "drop by" and "drop in" mean the same? are there some differences?

I was studying phrasal verbs, and I was curious about the difference between "drop in" and "drop by." I looked them up in the dictionary, but they seem to mean the same.

From the freedictionary.com, it says,

drop by: to visit someone casually and without being invited (I'll drop by at his house on my way home.)
drop in: to arrive informally to visit someone (Do drop in (on me) if you happen to be passing!)

If there is slightest difference, can you explain?

Thank you in advance

drop in, drop by and drop round all mean essentially the same.

I can't perceive the slightest difference between drop in and drop by. But drop round is purely British, never heard in North America.