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This is intentional, even if unexpected, as shown in line 152 of the link (turn on line numbers in the settings menu). I agree that following PEMDAS would make more sense.
Edit: it's the equation below "Although if you want to find roots higher than 3..."
MIL-HDBK-5 [1] is a good publically-available source for strength allowables for several aerospace alloys, including multiple directions relative to the grain for some of them.
The first relevant example I found was on page 3-86, extruded 2024, 2.250 - 2.499 inch cross-section. For ultimate tensile strength, F_tu, the L (in the direction of extrusion) allowable is 57 ksi, while the LT (perpendicular to the direction of extrusion) allowable is 39 ksi. That's a 30% drop in strength.
[1] http://everyspec.com/MIL-HDBK/MIL-HDBK-0001-0099/MIL_HDBK_5J...