There are a few reasons cant get 90% MA
either ur DSR too low , so they cut the MA to obtain meet minimum DSR requirement (case to case depend on applicant)

or
overvalue.
bank may exercise a ceiling MA for certain project if they feel that the project is overvalue. if that is the case, bank will across the board approve at the same max MA.

certain bank also calculate MA base on the price after discount.
which resulted is a lower MA.

of coz u still will see why A&B have same income but B get higher MA?
that mayb due to few reason,
joint applicant?
pledge FD
earmarking of account?
and sum case, guarantor.

hope it helps.
Adios