If I say I took 5 hours for a century, I mean I started and then 5 hours later, I was done. This is hypothetical, you understand, for me, the 5 hour century is like a 4 minute mile, it's something to be talked about rather than achieved.

If I say I averaged 20 mph for a century, I mean when I got done, my computer/speedometer showed a 20 mph rolling average, even though I might have taken a 2 hour break and eaten lunch at the midpoint.