In 1928, Bentley had a problem with drilled frames at Le Mans. At least one of their cars had to retire with a broken rail. I would guess they were balancing lightness with durability, and hedged their bets by using different patterns. Bentley whupped the SSK's at Le Mans in 1930. The Bentley's had continuous supercharging and forced the Mercedes to blow out their headgaskets by using too much boost.