I've seen this happen on American style keyboards, which I attribute to A being frequently used (3rd most frequent letter in English), but is typed using the weakest finger of one's less dominant hand (assuming you are right handed). It's the confluence of these factors - frequently used letter typed using weakest finger of weakest hand - that can exacerbate escapement issues already present in the machine.

Ideally, those machines should be serviced; but in the interim try index finger typing with your left hand while touch typing with the right, which I find helps a lot.

I haven't seen the guts of a Triumph yet, but since you mentioned a Royal KHM, I have one, I'll offer a tip.  This is a WAG (Wild A**ed Guess), but I would look at the escapement.  A sticking rocker, in a Royal it is the piece that causes the trip at the escapement wheel and is activated by the key action through what is called a U bar. If it has the slightest bit of sluggishness, it may allow a 2 space advancement instead of one. This also seems to be key related, and the left side at that, I would also look at the U bar.  It should have NO side to side slop at it's pivots; but be free to turn on them.  Also see if the links from the key levers are free, straight, and move the U bar freely and the same amount for each of them.  Sometimes I find faults by just exploring, and guidance from the repair manuals at Richard Polt's website at xavier.edu.  With those and some basic tools, I've saved the expense of a repair shop a couple of times.