Is it normal that the Avios/£ ratio (or value-per-Avios) changes depending on the size of the discount you use on a ticket?

I had always assumed they would be roughly the same (with a bit of rounding) but my recent booking suggested I could spend anything between 130 Avios/Pound of discount for £20 discount and 185 Avios/Pound for a £400 discount. (I went for somewhere in the middle).

Is it always the case that it costs more Avios per unit of discount, the more discount you use? Because that’s the opposite of what I thought it would be. (Also, it’s entirely possible my maths is off and I need more coffee...)