Adding A Filter with a Zero value

[cyclonebill]

When I add a filter to footage on my timeline, for example the brightness and contrast (bezier) filter, it defaults to both 0 for contrast and brightness (which is fine). However even set to zero, there is a noticeable effect on the image once the filter is added. If I uncheck the filter, it reverts to a darker image, re-checking it makes it brighter again.

So....I'm guessing zero does not really mean zero. Is this normal?

edit: I'm noticing that adding any filter with a zero value does (makes things brighter)- is this normal?