Slight Drop in Brightness

[vol.2]

1- sources are typically different black levels
2- sometimes takes a bit for a tube to wake up after leaving it off for awhile

In addition to both of those suggestions from tv nut and electric m:

3- even from the same source, the black levels are often different from the point of origin. For example, if you're watching some cable TV, the exact black level (the point at which black is true black) can easily be different on different channels, and even within the programming from the same channel. The copy of Gilligan's Island they are using could have a different black level from the rip they are using of the Rockford Files or whatever.