That is just how the award tickets work. I am most familiar with the AA program, and know that the award tickets to Australia have a very wide range. All of these are planAAhead first class from the USA to Australia.

BA: 260,000 miles
CX: 140,000 miles
Qantas: 105,000-115,000 miles

Most would argue that CX first class is the best in the world (I used this award last summer). It doesn't make much sense to use the BA award unless you want to stop in London on the way. I went ahead with the CX award just to be able to see their legendary first class and the wing.

As for your case, you can't go wrong with BA. The difference in miles required is not based on the quality of the service given, but rather on some formula derived by the airlines.