Because the wiki is all wrong.
Diamond:
0.016307%
0.033415%
0.052824%
0.067480%
0.085338%
0.081086%
0.081136%
0.085188%
0.081186%
0.079085%
0.077535%
0.079886%
0.070281%
0.041419%
0.007854%
0.000000%
0.000000%
Redstone:
0.145615%
0.266870%
0.384122%
0.520083%
0.661997%
0.654193%
0.647040%
0.648841%
0.659595%
0.648991%
0.660346%
0.657645%
0.597468%
0.336901%
0.066580%
0.000000%
0.000000%
Lapis:
0.003402%
0.007053%
0.013456%
0.022260%
0.033265%
0.037467%
0.040168%
0.040618%
0.044770%
0.050523%
0.059927%
0.065429%
0.067130%
0.071532%
0.062328%
0.057476%
0.054624%
Gold:
0.020959%
0.036416%
0.053974%
0.071732%
0.088390%
0.091241%
0.095093%
0.097644%
0.101445%
0.092291%
0.092441%
0.092591%
0.086339%
0.089040%
0.095093%
0.093792%
0.094592%
Iron:
0.100295%
0.187884%
0.286128%
0.380320%
0.455154%
0.460456%
0.453153%
0.444849%
0.447450%
0.469410%
0.464308%
0.461957%
0.461207%
0.445650%
0.447951%
0.459006%
0.463558%
Coal:
0.174478%
0.334950%
0.492671%
0.657294%
0.832623%
0.829772%
0.819767%
0.827671%
0.850281%
0.826920%
0.788203%
0.787603%
0.797107%
0.797357%
0.796807%
0.800809%
0.807012%
All these stats start at layer 1 and go up to layer 17.
Basically, if you want diamonds and hate lava, mine at head level 12. If you want diamonds and don't care about lava, mine at head level 7. If you want lapis and don't care much for diamonds, mine at head level 14.
This is an analysis of 36 million blocks over 18 layers, so i'd say it was pretty accurate.