I suspect the difference in sizing is more based on what other sizes that style comes in - if the other options are newborn on up, it's probably sized 24m. If 2T is the smallest and it goes up to 6X, then they'll call it 2T.
This is my experience as well, and it also explains the less baby-ish look of 2T clothes since they're designed more for preschoolers than babies. In the toddler sizes (2-6) the T designation seemed to mean extra room for a diaper/pull-ups in the seat.
DD is big for her age (4.5 yrs) so has always been 6 months to a year or two head on sizing. She also runs warm, so we have gotten a lot of use out of summer clothes and almost none out of warm winter clothes. Because of how much brands vary, at any given time she can currently wear 4, 4T, 5, 6/6X and I just found some 3T pants that still fit in the waist but are too short. Dresses, especially loose fitting casual styles, are probably the best because you can buy them big to start, then as the kid grows the dress could become a tunic. Same concept with buying tunics a size too big - they can be worn like dresses until they fit more correctly, then as tunics. We've gotten close to two years' of use out of some dresses/tunics this way.
Because of all the variation, the logical thing to do is have the kid try everything on, but that is laughably impractical. A better way is to find one item of each type that fits well or is roomy enough to grow into, then hold up others and compare the various seams (neck to shoulder, arm lengths, armpit to bottom hem, inseams in pants, etc).