Fruit trees - when is a first year a first year?

I'm always a little confused as to the instructions to remove most of the fruit from fruit trees in thier first year to allow the roots to develop and strengthen. The trees when purchased already have a few years behind them, so why when you first buy the tree is it considered appropriate to strip the thing (or is it?) as its its first year?

yes,  it's the first year after you have planted them 

last year I planted 3 apple trees but hadn't heard of the need to strip them of fruit in their first year so I just left them to get on with it. 

Not much fruit was produced (none on one, only 2 on another and 3 on a third) and last winter I planted 2 more apple trees but these fruited really heavily.  I have pulled the young fruit off the two new trees as I have since discovered the need to do this,  but last years trees are setting away like mad!! 

It looks to me that the lack of fruit in my original trees in year one (i.e. the year I planted them) has helped them to concentrate on establishing themselves and year two onwards seems to be a lot more productive!

If you dont mind a reduced yeild then carry on as normal until you can get it in a final 'resting place'    It doesnt need to build up a root system if you are eventually moving them Id say