Most digital multimeters do not measure true RMS voltage. Also, all transformers introduce a certain amount of harmonic distortion. This distortion is called hysteresis or "crossover distortion" and will result in a different peak voltage for a given RMS voltage than the nominal 1.4 ratio. So it's not uncommon to get a measurement error when reading a transformer's output using a typical peak-averaging multimeter.
However, you haven't been particularly forthcoming about your actual setup. As Bob and John T pointed out, if you are reading the DC output of a rectified AC signal, the voltage is not going to be the same as the RMS voltage but will rather be the PEAK voltage of the signal. For a 24 volt rms sine wave, the peak voltage is around 34 volts. It's details like this you need to understand before trying to roll your own battery charger.