AM4378: Power for DDR3 chip vs. DDR interface pins

I am working on a schematic for a custom board using the AM4378.  On the AM4378 General Purpose EVM Rev 1.5B schematic, the four DDR3 chips (sheet 6) are powered by a net named VDDS_DDR, which originates from VDCDC3, but the VDD_DDR pins of the AM4378 (sheet 3) are powered by a net named VAM437X_DDR, which originates from VLS1.  Why are there separate power rails if they are both running at the same voltage?

Thanks in advance for your help.

Jan