The reason is some fuses are rated continuous, like older glass fuses, some like Blade fuses are rated as blow ratings.
The rating of wire is always continuous, a 17 amp cable wille asily supply temporarily 25 amps top blow fuse.
Best way of working out amperage cable need is V/I X R, or watts /volts = amps.
18 AWG is 0.823 mm/2, 1.624 circular mm.
Additionaly thinwal cable carries around 35% more current than std insulation at the same thickness.
tabetha