Different folks and systems place the ratio of WLL : Tensile at different points.
For example dragging wouldn't have such a high margin as overhead lifting etc. Also, you might only be able to load a line at 90% tensile 25x, but 1000's of times at 50% etc. Mostly these are static loading conditions considered. Then we have environmental 'degrading' of use or non-use (sun, air, gas/oil, feline urine etc.); then there is grit that can get into lines and be worked back and forth on loading stretches to tear up lines/cut threads inside. So, different people, environments and industries might place different ratios on lines..and some manufacturers themselves might be more conservative to leave more 'headroom' to their quoted nos.
But, yet another factor to consider; would be the needed dynamics/ elasticity of a line under dynamic loading conditions. This usually is taken from line first in it's life span. Also, there are different ranges of recover -ability of the elastic hysterics. Part of the elongation might be unrecoverable, some might take minutes or hours to recover, and some is immediately recoverable. So, what worx 1x, mighty not another, for these (t)reasons; or even matching lines bought at same time, used seemingly same. 1 line might have more of the elasticity 'beaten' out of it permanently or even just temporarily. So, on a dynamic loading; 1 line might break, or even just not buffer the loading the same (so takes more loading on all connected points like tighten knots more, more likely to overpower support etc.).
Also, if elastic dampening is needed, tensile and length (besides construction and materials) must be considered. Because the longer the loaded line part, the more elasticity that can be granted. But, also a higher tensile line of the same construction and manufacture will give less dampening/buffering/elasticity to the same dynamic loading. Now, that means; that a lower tensile line will have a lower WWL to the static part of the loading, but will have more elastic dampening "WWL" to the dynamic part of the loading! So the tension wouldn't be as great in a dynamic loaded line of less tensile under same conditions. this also applies to pulley giving 2:1 support; each leg of support is less dynamic to same impact (so more bounce in SRT than DdRT). And you only count the length of line in a leg of support to calc elasticity; so the 'extra' line doesn't help in a 2:1 vs. 1:1 support.
