[Amps] Weakness of first two letters of call signs

[Jeff in Mali:]

In my pileups, I find that people often send their call signs only one time. Most call signs are of the form XX#YY or XX#YYY. By the time their sending gets to the #, their signal is strong enough so that I can copy the rest of the call. 
i am wondering if it is a common property of amplifiers that they take a finite time to reach full power and that as people send their call sign, the first couple of letters are being transmitted at less than full power?
Do people in the group have any experience with this phenomenon, or is it just my imagination? 
I try to get people to send their call 3 times (QSZ 3), but someone has convinced a lot of operators that once is enough.  
73,
Jeff TZ4AM