The green channel carries far more of the relative-luminance weight than the blue channel.
Contrast is a relationship between two colours, not an intrinsic brightness score stored in one colour.
The stronger black-or-white pairing is a useful default, while the rendered component still needs accessibility review.
The full explanation
A colour can look bright because one encoded channel is large and still have less relative luminance than a duller-looking colour. WCAG contrast starts by converting sRGB values back to proportional light, weighting the channels and comparing two luminances. Average RGB and visual adjectives answer different questions. Read the complete guide for the reasoning, examples and definitions behind this answer.