Regimental sponsorship

I was under the impression that once you get to RMAS, "all bets are off" (to quote the Regimental Secretary of my sponsoring regiment). However, I've just started reading 3 PARA (Patrick Bishop) and he writes that:
B Company was commanded by Giles Timms... the artillery sponsored him through Sandhurst... It was only at the last minute that he told his sponsors that he would not be joining them. 'I got a hard time for that, for disloyalty'. (page 18, if anyone really cares for the reference)
This presents an altogether different image of sponsorship to the one which I'm familiar with. Does anyone know what the crack is? Is it that there is a difference between theory and practice, or in how this is applied between different Regiments / Corps? Has practice changed in the last few years?

I got told the same thing this week. Apparently it has recently changed, hence the difference with Timms, and when you reach Sandhurst, "all bets are off".
Hello chaps,

I have just been offered sponsorship from a line regiment and in the letter the Regimental Secretary wrote that I did not have to choose the regiment and that they did not have to choose me.

As far as I understand, under the new system, in term 2 you make your two choices.

Each regiment will then have a batting order and you will then get assigned a regiment/corps dependant on this order, your order of preference, and what your competitors have chosen.

You dont have to have visited or done anything at all with the regiment or corps you choose; but obviously the more you have engaged with the regiment or corps before term 2, the higher you will be on their batting order.