I know the design engineer who came up with the X500 seriesRespectfully, it seems to reason the GX255 is more analogous to the hydraulic X500’s being that it shared the frame of the LX yet had 300 hydraulics/transaxle.
X500 & X520 was a replacement for the GT series
X534 & X540 was a replacement for the GX300 series
Deere wanted to streamline the belt driven Garden tractors into one series
They used the same frame for the X500 series which is very similar to the GX300 series frame design. Then in turn wanted to streamline further as they had done in the Past with the LX series lawn tractors sharing the same frame as the GT series garden tractors.
The LT & LX series were combined to Form the X300 series LT used a K46 LX used a K62 Deere decided they Only needed a K58 so the K62 was replaced with the K58 because at the time Tuff-Torq offered More Options with the K58 then the K62
But it was More cost effective to use the same frame design for both X300 & X500 series instead of having 3 different Frames LT, LX/GT & GX frames. They could further steam line the attachments Because the LX series attachments would work On the GX300 series But the LT attachments only fit on the LT series Ie the LT100 series Blower Was Based off the L100 series blower