hello
mastering for singles can be different for singles and for albums and even for promos.
sometimes it will be the same master and sometimes its best to have them sound different because they can be a solo thing and not dependent on the whole album flow -
with an album you want the songs to fit with eachother unless on purpose you want to have them sounding different then the other songs.

in singles you can have a different vibe and tone to each song - and that way give it a different vibe which can benefit in promotions and radio airplay or myspace singles

so its normal to do different mastering for singles and albums