There are a few of reasons for this. The majority of the fleet is A319s and A320s, which are very similar in feel and eye height for the flare, the A321 although very very very similar is a little different, the longer body length means a slightly higher eye height for the flare. If you inadvertently revert to what you know best, ie the eye height for the 319/320, then the 321 flare is a little late and can be a little firmer. The speed is not really much different a few knots maybe, but it is weight dependant - obviously the weight at touchdown of the 321 is usually higher than that of the 319/320. A few knots can feel a lot more.

Then you come to the A320/321 Neo, these aircraft came with a Lift Improvement package - small aerodynamic changes to improve margins of lift, ie you can land slower for a given weight, which is important because the engines are heavier. Essentially, although the margin to the stall buffet stays the same, due to the larger frontal area of the engine there is more drag. this matters little most of the time, but in the flare, once power is reduced to flight idle, the aircraft slows down quicker due to the greater drag. This means the lift decreases faster than the CEO aircraft and settles onto the runway in a different fashion to the the CEOs.

Given that the flare is one of the few areas of flight where artistry plays a part, the finesse of the pilot depends on many things on the day, one of the most important being what one is most familiar with. Given there are still more CEOs than NEOs, most are more familiar at a gut feeling level with the flight dynamics of the CEO. There are more 320NEOs than 321NEOs so the logic extends further there as well.

Despite all these subtleties, the landings may feel more dynamic but they are still safe as houses, just maybe a little less finessed. As the old saying goes, any landing you can walk away from is a good one, it’s a bonus if you can reuse the aircraft.