If this is the case, I would be interested to know why in a technical sense.
So would I.
The only thing I can think of is that at least two different methods are used to achieve the results and that different licenses enable to eh different methods.
I'm not sure Siglent would be keen on letting use know.
Perhaps there is a clue in the specs from the data sheet. For 100MHz, 200MHz and 350MHz the ratio between quoted BW and risetime is 0.35, however for 500 MHz the ratio is 0.4.
My guess is you could go directly from 100 to 350, then successfully install the license for 500MHz.
Does anyone know what VGA is used in this scope?
Edit:
Seems it's a LMH6518. So no real help there as all BW options are simply programmable up to 900MHz. All the lower BW would be implemented with first order filters internally. The only clue is still in the .4 ratio from the Siglent data sheet. Maybe the 350 MHz is selected via the VGA and the 500MHz is some kluge on that rather than enabling the 650MHz filter in the 6518. Perhaps a high pass filter in the jfet front end to boost gain at higher frequencies?