Light bounces. Every wall throws its own colour onto every other surface, so four walls of a colour read stronger and warmer than the A5 patch you judged it by.
A tester patch is surrounded by the old colour, which is usually paler, and it receives almost no bounced light from itself. Once the whole room is that colour, the walls light each other, the ceiling picks up a cast, and the effect compounds — saturated colours gain most.
The effect is weak for near-whites and greys, which have little colour to bleed, and strong for saturated ones. It also depends on the room: a small room with a single window bounces far more light between opposite walls than a large one with glazing on two sides.
Estimate with the rule, then check it against the calculator that models it properly.
Open Room Colour Visualiser →Light bounces. Every wall throws its own colour onto every other surface, so four walls of a colour read stronger and warmer than the A5 patch you judged it by. A tester patch is surrounded by the old colour, which is usually paler, and it receives almost no bounced light from itself.