In Norway, don't know about the rest of Europe, you aren't allowed to state something weighs x grams, you have to state that it weighs x grams +-10%
That's because materials vary batch to batch, among other things.
I would say, just because the USA doesn't have such a requirement of honesty in weight listings, doesn't mean that the variable weight issue isn't also present here.
You're well within that range, as you're finding, the weights vary, somewhat randomly.
I just got a prolite regular, and it's about 30 grams more than the 16oz listed, ie, 484 grams, ie, well within the actual range of error in the specified weight. That makes mine within +6% of listed weight, not too bad. But it could be their new batches of materials are heavier by a touch, since nobody so far here reports lighter weights.
The only way you can actually see if you can find one that weighs x grams is to weigh the packaging, all of it, then go to REI and sit there with a digital scale weighing all the prolites.
Or you can not worry about it, and accept that just because companies don't list the actual range of weight variation doesn't mean that that range doesn't exist.
On the bright side, my prolite small weighs roughly the weight it's listed at, 11 oz.
Personally I'd rather have more than less material in my sleeping pad, maybe a bit more glue, or whatever they use to bind the stuff.
I have a tarptent that's close to 10% over its listed weight too by the way, but that's how it goes, fabrics aren't high precision I guess.
If I had to guess, I'd guess that thermarest's plant in where-ever it's made, got either fabric or foam that is slightly heavier than last year's batch. I'd further guess that most fabric based outdoor gear will see a batch to batch variance, or year to year, particularly if they re-source the main materials. I've seen a variance in 1.4 oz / yd silnylon for example of from 1.35 oz/yd to 1.6 oz/yd, no indication anywhere on the order pages that this variance existed.
