I think it's definitely a measurement issue. I don't think a server of Sampras' quality could possibly increase his fastest 1st serve by a full 10 mph between the ages of 23 and 30.
I suppose it could be possible that he was spinning it more on both 1st and 2nd when younger, which would mean that his accuracy got so good as he got older that he could hit flatter and still keep his percentages up. But this seems unlikely.
So, by the numbers, it looks like the measurement method changed sometime between '96 and '99. Should we really be adding roughly 10 mph to serves clocked before '99? This would make sense. I remember seeing some serve speed stats for Becker at Wimbledon and being somewhat shocked that his numbers were very similar to Federer's typical tournament averages: Fastest 1st ~ 130 mph, Average 1st ~ 116 mph, Average 2nd ~ 95 mph. After seeing that I thought that Becker may have had the most overrated serve of all time, which is a silly notion easily disproved by watching a match like the '96 Masters final. Adding 10 mph would give Becker: Fastest 1st ~ 140 mph, Average 1st ~ 126 mph, Average 2nd ~ 105 mph. Is that more realistic or less?
But, given that Federer's numbers in 2001 are nearly identical to those he posts today, I think we can be pretty sure that Sampras' 2001 numbers can be compared directly to serve speeds of today's players. And by that standard, his 121 avg. 1st is very good and his 110 avg. 2nd is simply amazing.
And yet, despite that incredible 2nd serve, his career 2nd serve points won percentage (53%) is lower than Federer's (56%), whose avg. 2nd serve speed is nearly 15 mph slower than Sampras' was.