Yes. The primary only beers seem to be less carbonated and very little head on those beers.
Then your beers not carbed yet. Length of primary has NO affect on carbonation level. If your beer is less carbed then you're doing something different or you are not waiting long enough for your beer to be carbed.
There's no logical reason for length of primary causing this....I've been long primarying on here probably longer that anyone, and my beers are no more or less carbed than the beers I occasionally secondary.
Carbonation has to do with the sugar you add AFTER your beer has finished fermenting and clearing. It's no different if you long primary for a month or use a secondary. The beer is really the same.
What you do prior to carbonation in terms of this is really null and void, it's what you do at bottling time and after that determines level of carb.
Are you comparing higher gravity beers that take longer to carb and condition with lower grav beers?
If you judge an ordinary bitter with a low grav at 3 weeks, that is probably finished carbed, with a higher grav beer like a Russian imperial stout at 3 weeks (which may take 2 months to carb in reality,) the RIS will APPEAR to be less carbed, simply because a RIS takes longer.
But none of it has anything to do with whether or not you did a long primary or a secondary.....
One caveat to this is, if you never take gravity readings and bottle after only a week without using EITHER a long primary or a secondary your beer could indeed appear to be over carbed,
because fermentation was probably not complete before you bottled. Fermentation is going to continue of the still unfermented sugars AND the fresh sugar you added, so the co2 level would be higher and the beer would be carbed more......and you could be getting bottle bombs as well.
That's why all those old homebrewers use to get bottle bombs back in the day because they didn't leave sufficient time for fermentation to be complete, or use a hydrometer...they added more sugar to beer that was still fermenting and trapped it all into a bottle.
Caveat two is similar to caveat one. If you had a stuck fermentation and bottled, it could appear the same way as caveat one for the same reason, fermentation kicks up again in the bottle.
But under normal circumstances, regardless of whether you racked at two weeks into a secondary for 2 weeks OR kept it in the primary for a month, the same beer will not be any more or less carbed than the other in this instance....