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Where did that come from?????
For your information (read slowly and carefully):
Bolt qualities range from 4.6 to 12.9 . Standard socket heads are in the better qualities only, and have 8.8, automotive grade is 10.9, special grades have 12.9. Most prop bolts are 8.8!
The first number denotes ultimatebreaking tensile strengthlimit/10, the second number is a multiplier for wherethe material just starts to flow (Hooke's law). Hence the number never can be 10, or the material would have no stretch at all, but only strain elongation within Hooke's law (0.2% strain limit, not resulting in permanent stretch). It would snap without warning.
example:
8.8 has 80N/mm2 ultimate tensile strength and 8x8=64 N/mm2 Hooke's law tensile strength (the safe limit, before serious deformation starts) That is why you should use screws, that are torqued beyond the stretch limit, only once, like in car clutch plates.