Resolved Question

the answer is always 1089 when you do this: choose whatever 3numerals containing number from 1 to 9 ... e.g. : 426 ok?
and now... write this number from backwards.. you will get 624, right? and now subtract these two nubers: 624-426.. you will get another number: 198 .. do the same with this one: write it backwards and subtle these two: 891-198... you will get 693.. and now write it backwards and ADD these two numbers... : 396 + 693.. and you will get 1089 !!! I tried it with another numbers.. it is the FACT !!! always 1089.. WHYYYYY???
I got this task from my boyfriend.. he still hasnt explained me why it is like that :p )

I don't have the solution, but I can point out a couple of interesting features that may shed a little light.

(1) In almost all cases, you don't need three steps, you only need two. Once you select your first number and do the first subtraction, no matter the number, you will wind up with either 693 or 198. Either of these will get you to 1089. You don't need that third step.

(2) Except that you do need the third step when, as koyptakh points out, there is not a sufficient difference between digits. Meaning, basically, 6 and 5. For example, 625-526=99. In that case you need a third step. 990-099=891 (which is the reverse of 198, and therefore gets you to 1089).

(3) 9 is the magic digit here. 1089, 693, and 198 are all divisible by 9. 9 does similar magic things elsewhere: try that first step with a two-digit number instead of three. When you do the subtraction, you will always get some multiple of 9. Perhaps because there are 9 digits total, not counting zero. Perhaps because 9 is just the square of 3, which is a simple number to manipulate.

Somehow, when you do that step with a hundreds-place digit -- that is, you are using 3 unique digits -- it drastically narrows the possible result when you do the subtraction. From there, it's a quick jump to 1089.