The I-238 number was specifically requested by the State of California so it could match the California Streets and Highways Code
The eastern end connects directly to the north end of CA-238.

at the time of the numbering all three-digit combinations of I-80 (the primary two-digit Interstate in the Bay Area) were being used in the state.
They could not use I-180 because there is a CA-180 in the Fresno area. At the time there was already an I-480 in SF, since decommissioned. Their code only allows different types of highways in the state with the same number if they are directly connected.