Its a few years since I claimed but having asked a very similar question I was told by those in the know that the journeys are return ones. The number was derived from assuming that you will take some leave each month and therefore to assist in reducing paperwork each time you do take leave they (the system) averages out your claim. This means that you do not have to refill/adjust claims each time and seemed sensible to me. This pre-dates JPA but I assume that the rules should in the glorious bedtime read of the JPA JSP.
The 40p & 25p rates are something to do with tax rates and what the tax office says is acceptable.