_score value doesn't match the return value of a script_score, is this a bug?

Hi, I have a script_score that returns a number but the explain shows another number in the score value of that script

        { "script_score":{ "script":"1443150000000" } }

The explain shows:

value: 1443150040000
description: "script score function, computed with script:"1443150000000"

Is this correct or is a bug?

Thanks, Claudio.