You must understand the difference between "deployed" and "mobilised". The first is where you are sent, be it on a two week exercise or something operationally. Mobilised is a legal state of employment were a reservist is brought onto the books of the regular service.If I blindly rejoined as you say, and then got deployed. If the pay didn't match my employer's, I'd come back homeless, with my wife and kids on the streets.
Where an individual is mobilised, either by intelligent selection or bulk order, the full effect of the RFA is brought into play. Your civilian status is considered and calculated on arrival at Nelson (I have heard of other service centres) and all your expenditure and income is considered. An example of expenditure might even be, for a single person, cost of pet care.
The MoD is not permitted to mobilise you at financial loss - stand fast a few Harley Street Consultants - and the calculations for your mobilised pay will use this as the bench mark.
Finally - Your protection of civilian employment is very very similar to that of a lady on maternity leave, in that your job (or one similar) must be waiting for you on your return and that you must not be treated unfavourably as a result of your reserve service, mobilised or not.