Impossible to guage mate .
When it comes to actual electrical wiring in houses etc cables have huge amounts of tolerance because they carry a constant current & would get warm/hot othewise.
Appliance cables ?? You'd like to think at least 20% tolerance over its rated spec.
Re the cables you're talking about ??
They will have an ampage rating printed on the plastic on the fugure 8 connector, I would honestly think that all old figure 8 style cables (IEC c7) are max 5a - you're talking about the old style cables that work on portable radios/ghetto blasters /ps2 consoles etc ??
https/www.amazon.co.uk/Metre-Figure-Mains-Cable-Power/dp/B0094JE9RS
I've never seen one with thicker guage wiring that I would assume would handle 13a (over 3000w).
I've also never seen an appliance with a removable cable that need 13a either (barring a 1800w PC PSU)
But yes the smaller the plug fuse , in essence the safer.