Free space of mdf and ldf does not match with database free space

In SSMS I saw file size related properties and found below details for one database. Here values does not match with other properties. Here size of the mdf, ldf and total size matches with other values under each window. But Available free space of mdf and ldf if added then it does not equal to Available free space shown in shrink database window and free space shown in database properties. This is true for any database. Why is it so? Please can anyone explain the logic behind this?

Under database properties:

Size: 91.31 MB
Space available:13.40 MB

Under database file properites:

mdf size: 17 MB
ldf size: 75 MB

under shrink database:

Currently allocated size: 91.31 MB
Available free space: 13.40 MB

under shrink file-for data file:

currently allocated size: 16.38 MB
Available free space: 12.63 MB

under shrink file-for log file:

currently allocated size: 74.94 MB
Available free space: 55.62 MB