The data for the 2182 look suspeciously good at 1 PLC and not that good for longer integration. There may be some hidden digital filtering or maybe not using simple AZ mode.
The Keithley 2182 gets extra complication from having separate auto zero settings for the ADC and input amplifier and some extra analog filtering. So it really depends on the settings and in some cases there is extra settling time. The NPLC setting alone can be misleading, even without extra digital filtering. The case with input chopping, but no AZ mode for the ADC is also a bit misleading, allowing good performance over a short time, but some 1/f noise and drift over the long run.
Working well at 1 PLC and not so good with longer integration is a problem with many Keithley meters - not just the 2182. My suspision is some hidden digital filtering that improves on the 1PLC noise at the costs of extra noise at 100 PLC.
The ADC in the K2182 has a potential for good noise, possibly approaching the K2002 and K2010 noise performance, as these are not that much different. In parts they seem to compromise INL for noise.
For shorter integration the 34420 is limited by the ADC: it is the same ADC as in the 34401 and for less than 10 PLC (and maybe even at 10 PLC) limited by quantization noise. It is kind of a very good input amplifier, but a relatively noisy ADC, especially at 1 PLC, but also the 10 PLC ADC performance is not great.