It makes more sense in the context of the text you're quoting (The Electrical Engineering Handbook Second Edition
By Richard C. Dorf ?) but it obviously is a case of going for coffee in the middle of a sentence.
While explaining the high-frequency effects of different kind of resistors, the text is comparing impedance (i.e. AC) to DC resistance in relative terms, which is a measure for the useful frequency range of a resistor (=highest frequency at which the impedance still is within the stated (DC) resistance tolerance)
The text explains that wire-wound resistors exhibit an increase in impedance over frequency (mainly inductive) and that composition resistors (which have low inductance) have capacitance due to the "many conducting particles which are held...by a dielectric binder".
It then mentions film resistors and explains that the impedance "...remains constant until 100 MHz...and then decreases at higher frequencies." while referring to the figure 1.7 that you posted.
So far it's all OK, but the author wants to say too many things at the same time with the next sentence. In my opinion he wants to:
- highlight the good frequency properties of film resistors (better than composite and wire-wound)
- explain that the impedance of film resistors degrades with higher resistance values (i.e. 100 kOhm ... 1 Mohm)
- explain that the rate of change of this "impedance as % of DC resistance" decreases with higher resistance values (*)
(*): this has maybe nothing to do with fig 1.7, the drop in |Z|/R should cap out at -20 dB/decade (while still capacitive)
I don't think he really meant to say that "the decrease in resistance decreases" but rather that the impedance as percentage of dc resistance decreases faster (i.e. the value Rac/Rdc decreases more) for higher values of resistance (i.e. it's worse for the 1 Mohm resistor than for the 100 kOhm resistor).
So that the sentence:
"For film resistors, the decrease in dc resistance at higher frequencies decreases with increase in resistance"
was maybe meant to be:
"For film resistors, the impedance as a percentage of dc-resistance at higher frequencies decreases with increase in resistance"
Which is still not very nice to read, instead I would write:
At higher frequencies, the impedance of high value film resistors degrades faster than the impedance of low value film resistors.