Throw a dyce - it's that way because the CAA section has a much larger ratio of retirees to contributing members than the NATS section.
The bonds are held to pay the current retirees' pensions as these require a stable cashflow. The more risky investments like equities are held to provide higher returns for the contributing members' future pensions as they don't need the short-term stability.
It's simply a case of prudent fund management when faced with differing investment goals, rather than any sort of risky behaviour.
What's more, if the portion held as equities were held as bonds, the underlying rate would have to go up to make up for the lower investment returns. So we'd probably have to accept an even lower cap - or worse still, have the scheme even more radically altered than what has just been accepted.