Alf,
It has to do with how your body heals. We are all different. If you look for posts about stricture, you'll see that several of our members have had the same problem. It can happen early and often, or late. It seems unpredictable.
The issue is how the suture to reconnect the urethra heals. If there is too much scarring, it blocks the urethra. Not a medical diagnosis (I'm not a doc), but I've had the discussion with my uro, just in case.
This is not rare (it seems from the posts of others), but you will need to be attentive to signs that flow is slowing down.