@swift some dry humor I guess, but mostly it is an attempt to point out a grammatical error by showcasing what exactly the error is. And to a very lesser degree, is also trying to soften the correction because some people might become defensive when given unsolicited corrections, but less so if its not obvious if the correction truly did not know (and sometimes the corrected person looks it up, finds out they were wrong, and are more happy about it as they feel that they get to share their newfound knowledge with a fellow, making it a happy experience!). Sometimes the attempt backfires though, and someone reads it as being mocked (which - as you seem aware of but I mention anyways - is not same as dry humor/friendly jab), so I only do it when the dry humor is enough to make me chuckle (and thus I deem worth the risk)...
And hey, it also makes helpful people like you show up, which brightens my day simply by seeing your existence
I can't say I am not selfish like that
(or is this just more dry humor? we'll never know)