[Langus]

Okay, from the manga I got the impression that the espada were around BEFORE Aizen took over HM. Aizen only took over HM recently, hence why there is still some obvious and some subtle resistance to his authority. It would seem from what's been said thus far, that Aizen's Hyogoku only made the pre-existing espada stronger and facilitated the quick creation of new espada.

So in relation to Nel, what you have is

Spoiler for Manga spoiler:

a high ranked spada who was kicked out of HM thanks to Noi for 100 years of their time (of course, we still don't know how time passes in HM and if it correlates to time in the living world or moves at an accelerated rate).

At the time of her removal Nel was ranked 3rd amongst the espada and the way Noi acts towards her and her confidence in attacking him would suggest that she has always been ranked higher than him. However, Noi also made a point to say that they've all grown stronger since she left and her rank is now meaningless (assumedly because of Aizen's intervention with the Hyogouku).

Be that as it may, Nel still stood her own against Noi in their first fight after her transformation and he certainly seemed intimidated by her power despite her 'apparent' weakness (in all honesty, had she not reverted back to her chibi form I think we can all agree she would have owned his high heel wearing ass). The only reason he was able to boot her out of HM to begin with was because A) Noi had help from Szayel who set things up and B) He attacked her from behind like a coward.

So really this boils down to two things: either Nel was always that strong or Aizen's Hyogouku didn't give them as much of a power up as Noi seems to think...

Just some food for thought.