Tricky Dicky presented the following explanation :
In days of yore when people often painted the putty fillet a different colour
to the rest of the window woodwork, painters would use a brush where the end
of the bristles were cut at an angle a guess at 15deg. This allowed easier
painting into corners and although these brushes were quite narrow 3/4€� I
suppose the same could be applied to a wider brush.

Cutting in brush??