Idiom 'rolling budget'

Business English Lesson, Advanced Level

ESL/EFL Test #155 [color=blue]“Budgeting”, question 10

Many companies maintain a budget, so that they are continually budgeting for this time next year.

(a) floating
(b) moving
© rolling
(d) shifting

Business English Lesson, Advanced Level

ESL/EFL Test #155 [color=blue]“Budgeting”, answer 10

Many companies maintain a rolling budget, so that they are continually budgeting for this time next year.

Correct answer: © rolling
[size=200]_________________________[/size]

Hi,
Why not choose "floating "budget? (though rolling budget is right, too, lol)

Thanks

.
The idiom is ‘rolling budget’; other than that, I can offer no rationalization.
.