like Diver0001 said above - it's a recommendation, rather than a rule as in: you are certified to this limit and if you go beyond it then that's you're own fault...
For a newly certified diver, this makes a lot of sense - however there is a lot to be said for folks who got their OW cert back in the day, or - in many cases I have come across - learned to dive before PADI or even NAUI actually existed (armed forces, usually). At the centre I worked for in Egypt we would take this into account - a lot of our older customers had indeed learned to dive before the agencies really took off, and had been regular divers ever since. and have dived deeper and longer than even the most experienced professionals would want to do today. They only got a OW license because the business started demanding them, rather like people who learned to drive during the second world war, drove quite happily for 20 years after the fact, were required to pass a test to carry on.
Depending on your location, some centres as per my place of work in Egypt will count experience along with certification in recognition of the fact that these divers have a basic cert, earned many year ago, but have a lifetime's experience behind them. Somewhat unfortunately, many centres stick by the rule that you can only dive to the limits of your certification, regardless of prior experience.
Australia is one of the most litigious countries in the world, and from my limited experience diving there, for all kinds of ridiculous Australian regulations, will only permit divers to dive to their certification. I agree with that to some extent, in that I don't want to see newbies doing silly things beyond their limits but on the other hand - there are those folks that have been diving since before the rules were ever invented...
Which is more of a ramble than an insight but that's the way it is. And back in 1990 the PADI OW limit was still 60ft/18m so possible misinformation from the instructor at the time.
Cheers
C.