datetimezone - how does it know?

I have a curious one here.  

I have two fields called FirstSeenDateText and LastSeenDateText, and I need to create an actual date/time from them. 

The code I use is: 
"FirstSeenDate", each DateTime.FromText([FirstSeenDateText], [Format="MM/dd/yyyy hh:mm tt"])
and 
"LastSeenDate", each DateTime.FromText([LastSeenDateText], [Format="MM/dd/yyyy hh:mm tt"])

and it seems to work fine, except when I subsequently change the format to datetimezone, some of them have an hour added on, and some don't (those which don't highlighted in screen shot): 

My question is: how does Power Query know which are in a different time zone, if the original data is only text?