Sure thing. In the case of Celebi. It has 100 base stats all round which is pretty decent however like all pixies suffers from the fact that while they are good in all regards they have no specialisation (Arceus suffers just a tad from this). Utilizing its great bulk to rake up say a Nasty Plot (A coveted move among the metagame.The prevalence of rain removes one of it's weaknesses in fire. A comparison too say Gardevoir a pokemon that is also psychic type that resides in the NU tier. Gardevoir lacks a decent ability trace is too reliant on on what your opponent has, synchronise could be useful however no one is going to will-wisp a special attacker and its much easier to just target it's very low defence, Celebi which often finds itself being specially defensive could potentially be poisoned so being able to lose that status by just switching is always useful (Being coupled with U-Turn complements that nicely). Celebi has access to reliable recovery in recover-once again very useful on anything fulfilling a defensive role. Gardevoir on the hand has middling speed by NU standards and would be out sped by the majority of OU with ease, and is outclassed by Alakazam as a Special Attacker or Blissey as a Special Wall. For that reason Celebi is clearly the superior pokemon. On the whole Pokémon reside in their respective tiers for these main reasons.
(1)- Base stat total. Fairly self-explanatory.
(2)- Type (Stealth rock has caused many flying, bug and fire dual types to reside in RU or NU like Moltres), and of course resistances and stab.
(3)- Move set- Coverage, power and boosting. Moves such as Quiver Dance have made all sorts of Bug Types actually viable. Volcorona would reside in a lower tier without it.
(4)- Ability- Making things such as Breloom and Scizor top OU with technician. Regenerator makes Ho-Oh who would otherwise suffer greatly as a wall (50% damage by just switching into Stealth Rock. Serene Grace makes Jirachi the most annoying thing to grace OU since Sand Veil Garchomp was banned.