Okay now that the solution is no longer a spoiler, it is really quite simple. TLDR:

When you pick a random point in time you are more likely to end up in the middle of a long block than a short one. (Think about it: There are blocks that are 1 second or less. How likely is a random point in time to end up in one of those?).

It turns out that the average block time you end up picking with this method is 2 minutes, even though the average length of all CLAM blocks is 1 minute.