I wondered that myself and never really found out why, but basically the width of the wheel doesn't mean that only one size tyre can fit on there. Some people on the show scene like their tyres to look stretched so they end up putting a 165 width across their wheel even though stock might be 185.

Dont know if I'm making sense here or not so I guess the best bet is to talk to the tyre place and they'll probably explain it.
As I said above I can guarantee that mine were 185/55/15 stock and I've now gone to 195/50/15 and you will be able to go the same way. Another user on here with a Sportline (eyeletboy) is running 195/50/15s on his I think. If you are unsure have a chat with the place that is doing your tyres and they should give you all the information you need. Also, yes a 50 profile will be a harsher ride than a 55 but it all comes down to how you want the car to look and drive and what is the most important to you.

I dont know if they do a 195/55/ tyre and that will be a bigger tyre wall than the 185/55 as the second figure (55) is the percentage of the size of the width, so 55% of 195 will be a larger figure then 55% of 185 if you catch my drift and your cab will look like a 4x4.
If you really want to be clued up on tyres have a look at this site. -->
http://www.carbibles.com/tyre_bible.html
Your choices as far as I can gather are stock of 185/55/15 so you will have a skinnier tyre but a comfier ride or 195/50/15 where the tyre width and profile will look better but the ride will be slightly compromised, I also found that going from 185 to 195 meant that my turning circle at low speeds was slightly worse. If you are running stock suspension you might also find that changing your tyre profile means there's more space in your wheel arches, thinking about it there are a few things to think about when just changing the tyre size ! Apologies for long windedess of my reply too.

I have some before and after pics of me lowering my car and it was on stock tyres of 185/55 so I can post them up if you like just to see the difference and the fact that going to a 195/50 will mean that there is more space in the wheel arches. If you need me to post them let me know, I dont think I took any when I moved to 195/50s though.