2 points here, firstly, the log in date etc. can relate ( if using computer, and not a DNA specific app), to your basic Ancestry account, which many people have for years before testing their DNA. On your DNA matches 'home page' the date relates to their DNA testing date or results available date, and not the date they first joined Ancestry.
Secondly, of course many, like me don't sign off regularly, so their last sign on/in date bears no relation to being signed in, in order to send a message, as you stay signed in to the service on your computer.
Jane