The ADS7828 has two address pins and can therefore be set to occupy one of 4 I2C addresses. Gravitech have presumably decided on one such address.
If you look at the data sheet on page 12 Fig 3 the address format is
10010 A1 A0 R
0x90 is
10010000
so they have set A0 and A0 (the yellow bits) to LOW on the PCB. (The LSB is a R/W flag.)
Rob