I met a dwarven sphere guardian for the first time in 5months and surprising it took me more than 20 arrows to kill it. It also dealt 1/3 of my health bar(400HP) per hit and it can do range and melee.
Now later on, i met a centurion which took me only 10ish hits and it only did minimum damage??????

I've found the oppsoite- guardians are (relatively) easy to take down, but I need to be careful with the Centurians.

Maybe centurians aren't leveled, but the guardians are?

Centurions are stupidly easy for close melee classes. They only have that steam attack for range which can be triggered and avoided, then rush them head on. They don't move much after that.

Spheres bob and weave constantly and that damn arm-bow is fire enchanted.

Might explain why. I always engage in melee, where that arm-bow is rarely used.

It sounds like a glitch maybe....

The hardest Steam Centurion I have ever faced was inside the room with an uneven floor in Shimmermist Cave, on a flat level surface getting in close is easy.
The Sphere in Blackreach is the nastiest Dwemer Automaton in the game, if it is not destroyed before it detects your character. The ranged attack of the crossbow seems to always kill in less than three hits on all of my characters. I am using Live Another Life, and I know how to kill it when entering Blackreach normally, but when using one of the start options in the mod, that Sphere is a pain to survive.

Dwarven Sphere Guardian has 453 health. Does it mean your bow/arrow is doing 20 damage? That doesn't sound right. Maybe you missed some shots. It does 55 regular damage and 82.5 power attack damage, which is reduced by your armor. Do you not have any armor? What difficulty level are you playing on?

http://www.uesp.net/wiki/Skyrim:Dwarven_Sphere_Guardian#Dwarven_Sphere_Guardian

^ This.

Sounds like your numbers are surprisingly weak for the amount of damage relayed. What level are you and what Perks (in addition to the what the person above me asked) do you have for Armor & Weapons?