tl;dr as SF said in a comment "...they always packed a little more hydrogen than oxygen (for that ratio), so that at the end of combustion they wouldn't risk running oxygen-rich (and as result engine-rich)."
If you are interested in details, please refer to this propellant loading table for the following explanation.
The numbers indicated by the purple arrow were the masses book-kept in the propellant load to guard against contingency situations. FPR was Flight Propellant Reserve and was extra propellant booked to guard against certain off-nominal cases. FUEL BIAS was extra hydrogen loaded to make sure that the system never had a oxygen-rich cutoff.
From the answer to this question: Is "liftoff mass" = "ignition mass"?
An important thing to know when interpreting this information is that there is not just one mixture ratio for the engines.
This is because the shuttle main propulsion system used autogenous pressurization. Some of the propellant entering the engine was vaporized and returned to the tank as pressurant - it did not flow overboard.
So there are two mixture ratios in play:
- "inlet mixture ratio" (MR) - the ratio between the flows of propellant that enter the engine
- "overboard mixture ratio" (OBMR) - the ratio between the flows of propellant exiting the nozzle
Since the pressurization flow of both propellants was ~ 1 lbm/s, but the inlet flow of LH2 was much less than the LO2, the OBMR was always higher than the MR. You can see these numbers in the upper right corner of the chart.
(Also note that the mixture ratio was programmable for the SSMEs and changed over the course of the program. This chart is for STS-5, which was the last flight of the original version of the SSME. See Taxonomy of the SSME.)
So if you wish to match the overboard mixture ratio, the line in the chart is USED AT OBMR, defined as
USED AT OBMR = LOAD AT SRB IGNITION COMMAND - UNUSABLE - USABLE RESERVES - SHUTDOWN CONSUMPTION - VENTED AFTER SSME VALVE CLOSURE
The FPR and FUEL BIAS are included in the USABLE RESERVES as propellant that is not planned to be used but is available for off-nominal situations. A lot of work went into safely minimizing the usable reserves because every pound of this propellant was a pound that could not be used for payload.