When classification constant is defined as 'height':
1, module doesn't respect 'Sort' parameter.
2. Results in 'Module execution failed', but result looks okay (taking into account 1.)
The name 'Sort' itself is confusing. From what I understand it just depends how we look at hypsometric curve, but usually all area is above outlet, and no area higher than highest point.
When classification constant is defined as 'area', relative height goes only to ~50%.
Effective number of classes for 'height' is +1, whereas for 'area' classification I got only 1311 when requested 2651 (I wanted 2652 in fact).
I attach the snapshot with SAGA's hypsometry plot and that from ArcMap. To get hypsometric curve in ArcMap, I multiplied all values by 100 and converted them to integer. It allowed me to get a table with elevation and cell count that I used in MS Excel to produce plot. Elevation was divided by 100 in Excel, of course, and cumulative area was calculated from cell count.
P.S. I used 1m DEM in ArcMap and 3m DEM in SAGA, but it shouldn't result in such behavior.