Why is the maximum PCBA Panel Size so small compared to single PCB size?

Hello JLCPCB support,

we wanted to order a pcb assembly with a v-cut single-layout aluminium panel 20x (4mmx375mm) + 2 guides for the v-cutter and fiducials, resulting in a panel of 100mmx375mm.

But the limits for panel-assembly (250mmx250mm) are way smaller than for single piece assembly (460mmx500mm).

Can you elaborate why it is like that?

Thank you very much