Basically, the figures/gifts change on the 2nd screen every time, and you can only ever get those numbers. Forget about the first screen, just go to the second, and try and get 2 different gifts. You can't.
Sorry to spoil the fun... but when you take away the two numbers, and end up with two digits that add up to 9 (eg 45, 63 etc) take a look at the chart, and notice that all the squares where the digits add up to 9 are the same.
Neat trick though, made all the better by having a new chart each time. Now we have to work out how she gets it wrong sometimes ;-)
That's the end of that thread then! The fun lasted all of 10 minutes!!!
Our accountant at work spotted it straight away while the rest of us were coming up with weird random theories on how it's done. Still seems quite clever when you are a total maths retard like me!