In maths, we call percentage difference or relative difference the ratio between the difference of the reference value and observed value, divided by the reference value.

Which give you :

relative difference = (x - xref) / xref

In this case (2.8 - 2.4)/2.8 = 0.4/2.8 = 0.14

Relative difference is an unitless indicator that takes into account the size of the value. For example if we considered now that the buff value weren't a 2.4 and 2.8 multiplier but a 222.4 and 222.8 multiplier, would you still call that a 40 % loss ?
Even if your maths would still give you the same result you clearly see that in this case the loss is negligible (0.18 % loss to be precise).