stake of unmatched bet is reduced when taking SP

I am playing with a very simple trigger but don't understand why the stake is changed when the unmatched bet takes SP at the start of a race.

Trigger: Lay favourite: $5 at 1.2. If unmatched then take SP.
When the race starts the stake is reduced to a random figure under $1. eg $0.93. Why doesn't it keep the original value of $5?
I should add that I am also backing the same horse for $5 at SP.

When laying at SP with Betfair you are laying to a fixed liablity. For example when laying fav £5 at 1.2 you are laying to a liability of £1. (ie 5 times 0.2) If actual SP odds are higher than 1.2 then your stake is reduced so that your liability remains at £1. 

If actual SP odds are 1.2 then your stake is your liability of £1 divided by 0.2, =  £5 stake

But if actual SP odds end up as being 2.2 then your stake is £1 divided by 1.2, ie  0.83.  This is correct behaviour in the way that Betfair works with SP lay bets.