just to share my own experience:
I got thru RSAF interview and stuff.. even the 02 sureties signed. However, I received a rejection letter few weeks after. No reason given. After comparing with my friend, I realised I had one "extra" day of medical. My "second" day of medical, they puffed some air into my eyes, then asked me to wait outside for 1hr, then puffed again, then see MO.. this goes on and on from 8am - 4pm. I thought this was normal until I found out that my friend didn't went through this.
However, for my SIA medical screening which was at EXACTLY the same medical centre. Exactly the same checkup, same stations, except for the checking of length of your leg, length of your arm, head size, etc etc which wasn't necessary for commercial pilot. I informed the MO of my previous RSAF medical checkup and queried what was that for, he didn't bother to explain and just replied that it won't affect my SIA medical.
Well, till date, it's still a mystery why I was rejected in RSAF but enrolled in SIA. Oh yah, on the side note, I have perfect eyesight (during then, not now). I was pes A in NS.
On a personnal note: I wouldn't say commercial's pilot medical are less stringent. Not everyone make it thru the medical for SIA, there are cases of candidates who passed every damn thing except medical, gosh.