Round numbers

The idiom of 'round numbers' is a strange one. In what sense is 100 (say) a nice round number ...? Surely it's characterised, rather, by an unusual flatness? The gorgeous snailshell curl of an irrational number, always progressing in an ever-tightening neverending spiral, converging on the perfect iteration, is a much better fit for the 'round number'. π, the roundest number of all!