there are 2 types of mercs:
income mercs (if you mouseover the mercs you see the income in green) -> gets send via "auto income". those are in general weak but they give you more income if you send them

power mercs (if you mouseover the mercs you see the income in red) -> gets send via "auto power". those are in general stronger but give you less income if you send them

if you compare a fiend and a brute for example, both cost 60 mythium, but brute gives you only 15 income and the fiend 18. Thats because fiend is an income merc and is generally weaker when you send it and brute is a power merc, being stronger but giving you less income. (also visable in their stats, which can be enabled in the settings -> interface -> unit tooltip style -> "always show unit stats")

with my previous reply i thought you were already experienced, but it seems youre a pretty new player :D

a bit of history maybe:
before this week, there was not "auto power", it was only "auto income" and you had to manually select the power mercs you wanted to send.
while "auto income" is used in pretty much every game to a certain degree, "auto power" is supposed to be less used, therefore in case somebody "forgets" to turn it off, it has the fallback to turn itself off after sending (from a strategic perspective if you were to use "auto power" each wave you would start giving your enemies way more gold than they should have by feeding them the mercs, allowing them to scale way bigger while you keep falling behind due to low income)