I noticed this also. For the first 15 years or so after fitting PVC gutters in place of the old cast iron stuff, all was fine. Then I had the roof replaced, and shortly afterwards the gutters started leaking; some damage was caused by the clumsy roofers (annoying because I specifically asked them to be careful, and to check them afterwards as I couldn't be there to check myself when they finished and took the scaffolding down), they did come back to sort out most of the problems but I did suspect that a bit of grit or dirt had found its way into one joint causing it to stick upon expansion and then only contract from the opposite end, leaving a gap for water to leak out and run down the wall. I got fed up of borrowing the neighbour's ladder to keep re-fitting the gutter in the joint, only for it to recur a few weeks later. This year I had scaffolding up again for another job, so decided to replace the whole lot, and when clipping it all together I found that there is now a longer overlap in the joints - something like 2cm, as opposed to about 1cm in the old guttering - presumably for the very reasons you outline above.
Also, conscious of the heavier rainfall we can now expect, I opted for the deeper guttering rather than the standard depth, so should be OK for a while at least!
If you intend to remedy this problem yourself, I would suggest hiring a tower scaffold (I had the loan of one the first time round) it will be much cheaper than the full monty, and quite easy but very safe to work off of. Should only take a day or two if you plan ahead and can crack on once started.
