Is it a mistake or standard procedure to lit the landing lights only on very short final?
It is not an ICAO requirement to operate landing lights whenever you are below 10,000 FT. The mandatory aircraft lights you will see in regulations is strobe lights, nav lights and anti collision beacon lights.

It is standard operating procedure at several airlines to only turn on the landing lights when a landing clearance is received by air traffic control. This is most likely what you have observed. The landing clearance is usually given at very short final, and so crews will turn on the landing lights on very short final (unless you are in FAA territory, where multiple aircraft can be cleared to land on the same runway at the same time ...)