Qerey on expiry date calculation

Dear all

I have to maintain total shelf life for material 5 year

when I maintain shelf life with Period Ind. for SLED D and Total shelf life 1825 days

suppose manufacturing date is 22.12.2015 SLED calculate as 20.12.2015 in MIGO

and when I maintain shelf life with Period Ind. for SLED M and Total shelf life 60 month

then SLED calculate 22.12.2020,

Why this difference period indicator D and M

Regards,

Vipin Thakre