Can anyone explain why FB uses all 3 in the OR query yet only 2 in the AND query (See definitions below)?

Thanks,
Tom
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CREATE INDEX I_TARGET_COMPANY ON TARGET COMPUTED BY (UPPER(COMPANY));
CREATE INDEX I_TARGET_FIRSTNAME ON TARGET COMPUTED BY (UPPER(FIRSTNAME));
CREATE INDEX I_TARGET_LASTNAME ON TARGET COMPUTED BY (UPPER(lastname));


select * from target
where
upper(company) starting with 'A' or
upper(firstname) starting with 'B' or
upper(lastname) starting with 'C'

PLAN (TARGET INDEX (I_TARGET_COMPANY, I_TARGET_FORENAME, I_TARGET_LASTNAME))


select * from target
where
upper(company) starting with 'A' and
upper(firstname) starting with 'B' and
upper(lastname) starting with 'C'

PLAN (TARGET INDEX (I_TARGET_LASTNAME, I_TARGET_FORENAME))