You are essentially insuring your salary when you take out income protection cover. If an illness, injury, or whatever makes you unable to work, you will be looking to replace it with your income protection cover.
It is a simple rule that the more you have, the more you spend. When life is good and you are earning a nice salary, the chances are that you will have higher outgoings. The gym membership, the subscriptions, and the nice clothes all cost money, and you will want an income to match that lifestyle.
The more of an income you wish to have, the closer to that 75% figure above, the higher will be your premium. The good news here is that income insurance premiums are tax-deductible, which may reduce some of those extra costs.