Calculating RC oscillator frequency
I think it's closer to 2.53*RC than 2.2*RC (Ref RCA ICAN-6267 - note that the complex formula (2) in that app note is wrong, but the test results are usable), but the formula you have should be basically correct for low frequencies, relatively high resistance and R1 > 2*(R2+R24). 

The resistance is probably still low in comparison with the output resistance of the 4049 (probably &lt; 100\$\Omega\$), but the input capacitance of IC1A is 15pF typically, so there's another RC in there (charge and discharge) with the 100K which will cause it to oscillate much more slowly.

Basically the equation falls apart at high frequencies. If you re-did the test at ~1kHz with C = 100nF you'd have much closer agreement with the simplified formula.