Same a/c - different range?

[Wayfarer]

I am confused over my 2 new leased ATR-72's here.  They have the same engines and seating Y64-STD but have range 780 nm and 930 nm.. They are not yet scheduled, so why the difference? Also among my existing a/c, the same difference of 150 nm!
I don't understand this..

[Continental Sky]

They have different MTOW/payload, go to aircraft page and you will see three different MTOW options with the same engine, 780, 930 and 970 nm range. Higher MTOW, more fuel, longer range.

[Continental Sky]

but I can't increase seating capacity to more than Y74 in HD config on either of them

Hover the question mark next to the sitting configuration in aircraft page, and you will see what is the maximum number of seats, regardless of MTOW option.

But then actual range will vary depending on number of seats - click small graph icon on the right of "Nominal still-air range" to see payload / number of seats vs range (chart opens in new window, click directly on chart to alternate between payload and number of pax).