The rule used to be that a guest replacement for a reserve rider had to be another rider who was currently in a reserve position for his team, so even if your missing rider had, for example, a 5.02 average, you had to use a guest who was holding down a reserve position for his club with an average lower than that of the missing rider, even if there were riders in the main body of a team with an average lower than the injured rider for whom the guest was required.
I recall that there were fans who used to moan comment back then that was unfair as should be able to use any rider as a guest for an injured reserve so long as their average was lower than that of the reserve rider he was to guest for......I think it was somewhere around 2009-ish that the rule was changed to how it still currently stands, obviously the majority of promoters back then were happy to change the rule, probably on the 'you win some, you lose some' basis on who might be eligible to come in as a guest.
To get over the fact that a reserve might be perceived to have 'easier' rides, the 5% home uplift does not apply to a reserve's average when looking to use a guest replacement.