There are a few things that could be going on.
First, some people use macros that can automatically bet a certain percentage of the pots. So the odd amount could just mean that it's betting 33.333% of the pot.
I don't use macros but I like to make odd bets on occasion. Basically, the weird bet in a tournament is to make the other players think.
Sometimes I'll do something like a 399 bet as I think it makes the other player think "why did he do that?" So calling machines might be less likely to call if they're stirred from their coma and curious players might be more likely to look me up to see *why* I was doing that.
When I'm in a more playful mood I'll do stuff like bet 1,111 instead of 1k, 999 instead of 1k, 1,234 instead of 1k, etc.
In ring games my bet can just mean that it's my closest approximation to the percentage of the pot that I want to bet. I'm estimating 70% of the pot, know it's somewhere between X & Y, and pick a random number in between. I *feel* like it conveys strength... like an experienced player is using macros. So I guess I'm more likely to do it if I'm bluffing. But if I do it once, I'm going to keep that pattern up for awhile.