400V Architecture can mean many things as claims are often made based on the maximum system voltage rather than the nominal battery pack voltage.
This plot of maximum and minimum pack voltage versus the nominal voltage was used to show the increase voltage range as you move to Higher Voltage Packs.
What you also see within this is the clustering just below 400V, just over 600V and around 800V. OK, this is benchmarking data and so you do need to squint, but there is a reason for this.
Cells in Series and Pack Voltage
At 400V nominal with NMC/NCA chemistry we typically see 96 to 108 cells in series.
108s at maximum cell voltage of 4.2V the pack voltage is 453.6V and this typically equates to the maximum system voltage that works with 600V silicon devices.