All I was indicating 30W was that even if the subsequent investigation says that traffic levels were in the pescribed limit, that an overload has still occurred just due to the SC saying he/she was overloaded. Overloading can sometimes be with very few aircraft, just a super-high complexity.
I wish we had sufficient staff to split all sectors at all times of day, but the shortfall is well documented on PPRuNe. The way sectors work is that they look at the demand for each and work out which combination for bandboxing works best. Some staff members can do more than one area so 'borrowing' one for a while can also be done, but not for an entire shift. To give you an example, the Dover/Worthing sectors can currently be split into six different radar seats, or 7 but procedures for one split (DVR high & low) aren't currently available. Now if S19/20(Hurn) is above flow counts as well as DVR and LYD bandboxed (S15/16 & 17) during a half hour period, but both cannot be split simultaneously due to staff, then it may be just S19 that needs flowing as the demand on S20 is low, and then DVR/LYD can be split. Hence your departure from EGBB/EGBE/EGNX (my guess at where you fly from) is free flow, but all those coming back have slots although you are all worked by the same controller. I cannot give you any technical reason why this seems to work better, it just does.