Platoons are limited to only 3 SPG per platoon. Welcome to Wargaming. Lastly, if every member of a Platoon has a streak of bad luck in their Battle Tiers, the Platoon's next match will get a "friendlier" battle level. Matchmaker WoT.
As with all classes in the game, while most light tanks share similar characteristics there are those which deviate from these norms.