Banker explained

A banker is the leg bettors anchor a coupon on — the 1.25 favourite 'just to boost the odds'. The arithmetic most slips ignore: a 1.25 price implies a 20% failure rate even at the bookmaker's own optimistic pricing, and that failure kills the whole accumulator.

Bankers also carry the worst value on the board: heavy favourites are where recreational money piles in, so their prices run below fair more often than any other segment. Adding one to a coupon usually adds margin, not safety.