Thank you for the thoughts and ideas, Boeroer and thelee
I started a new run, and played through Port Maje without hiring anyone, and without disarming any traps (I think). Then I went straight to Sandswept Ruins, and got the correct 280 exp per trap
As it turns out, dismissed companions at the Inn actually "consume" exp. To test this, I reloaded my save before entering the ruins, sailed back to Port Maje, hired Aloth and dismissed him at the Inn. Back at the ruins, I now only got 144 exp per trap. So at least for disarm exp, you get less if you have dismissed companions.
But here's the funny part. I reloaded, hired Aloth and this time, did NOT dismiss him. Went to the ruins together with Aloth, and now I got 226 exp per trap. More than when he is dismissed
So to summarize:
Solo (never hired any companion): 280 exp
With Aloth in party: 226 exp
With Aloth dismissed at the Inn: 144 exp
I know this is super specific (obsessive?), but I thought I'd share these results in case someone else is confused by this. Due to the number of traps in the ruins, the total exp difference is actually pretty large.
@Boeroer: That's nice, I hope you enjoy your trip. They are interesting cities (as is Berlin, of course). By the way, this is off-topic but I'm trying to play a Steel Garrote/Blood Mage with Offensive Parry from mostly your ideas in this forum. But I'm unsure about attributes; if it needs high INT, DEX, PER, RES and decent MIG (maybe 15?), then CON ends up very low. If you could share advice on this, or give an example of a good attribute spread, I would appreciate it I'm pretty unexperienced with the game mechanics, by the way ^^