Nope, guard is a level 2 common agent, Kele is a level 5 elite agent and will provide a higher boost. Enjoy the extra %.
Yes. Use a level 2 common = 73% Use a level 5 elite = 74% Your common agent screenshot it showing 73%, your elite agent screenshot is showing 74%. The purple border means an elite agent, and its also a higher level soldier. The calculation scales on the level and rarity of the agents used in addition to the bonuses for the stats.
I am by no means an Overseer expert having only just returned to Live and having just started using it but I see what you're saying and the implication to me is that there are fractions of percents which aren't being shown to us and some sort of rounding. Arbitrary numbers example: Imp Merchant (64%) + Guard (9%) = 73% Imp Merchant (64%) + Kele (10.5%) = 74.5% rounded down to 74% Imp Merchant (64%) + Guard (9%) + Rat (8.5%) = 81.5% rounded down to 81% Imp Merchant (64%) + Kele (10.5%) + Rat (8.5%) = 83% Giving you your 2% difference instead of 1%. Again, I have zero idea how the back-end of Overseer works but this was what came to my mind when I first read your scenario as a possible explanation.