leveling up

i have a question,

i am DMing multiple campaigns at this point with my friends and i have a recurring problem.

Every time the group levels up their tier, the power scaling goes through the roof. i get that thats kind of the point because they go from one in a million to operativs of fate in just 5 steps, but to me it is just to extrem while doing it in a campaign. one minute they are kicking some hive gangers arse and in the next scene after leveling up, they can attempt to kill god(s).

can you quickly summarize the deal with leveling tiers and ranks just for me to be clear that i didnt misunderstand it?(since englisch isnt my first tongue)

are they not supposed to level up? and what happens to the ranks? in the rulebook it says you usually get 3 but after a level up they reset to 0? what happens to the abillities that scale with rank? my players rightfully aksed me this and it sounds unfair

would apreciate if anyone can help me out here