Tickets are more expensive on the train. Those who don't know or forget (or lie about their ignorance) can still escape with a normal machine purchased fee.
But you won't get the regular fee every time. You have to get lucky. The conductor has to be lenient, forgiving, and most importantly having a good day. You just have to show reserved character.
Complaining gives you no chance at a reduced fee. Anger is the expectation. However, if you smile or just show a plain face you get a chance at saving money.
On top of this is always the variable of running late. Of course you didn't have time to hit up the machine. Naturally, you're frustrated but you just accept the extra tax. This route ensures you'll pay up more every time.
That is, of course, is all avoided with a monthly pass.