The problem in this case of OP is that the POLICY for V1 is not known to the pilot.
There are various policies that can be set for different configurations, eg FULL v ATM can be different.
Without having access to the config file your question cannot easily be answered.
As suspected by other authors, my guess is that the policy is set to use the V1 min for FULL thrust departures, but
not for ATM.
Regarding Balanced Filed Length performance of FMC, beware that this does NOT include obstacle clearance calculations, hence OPT may be different as it does take these into account, and others for that matter...
As often is the case,
john_tullamarine is the guy to ask for the details and has explained various things very well.