Engineers wear a belt and suspenders.
All construction materials are laboratory tested to determine their structural properties, such as the amount they stretch and compress under loading and the maximum load they will accept before failing. The test results lead to the designation of a formal design strength for the material, which engineers subsequently use in real-world structural calculations. However, design strength is always established at a point lower than that at which the material failed, to allow for variability in quality.
Manufactured materials such as concrete and steel are of comparatively uniform quality, and variability from piece to piece is comparatively small. A wood beam, however, might have come from a diseased tree, have reacted atypically to drying, or contain an unusual number of knots. Consequently, the design strength for wood is much lower than that determined in laboratory testing.
Engineers commonly build in additional safety margins by overestimating loads, rounding their calculations to the conservative side, and selecting a structural member larger or thicker than the one their calculations call for.
