In a recent PACER (Probing Attosecond dynamics with Chirp-Encoded Recollisions) experiment on ammonia that comprises a comparison of the high-harmonic spectra of the isotopes NH3 and ND3, the nuclear dynamics of the created ammonia cation is traced with a time resolution of about 100 attoseconds. For modelling the experiment the autocorrelation functions between the neutral initial state and the ionic wave packet are extracted from experimental photoelectron spectra incorporating a correction for the geometry-dependent strong-field ionisation probability. Good agreement is found between model and experiment, but in addition an unexpected maximum in the autocorrelation ratio is predicted by the model, however occurring at 5 fs and thus outside the experimentally covered time interval. In this work the autocorrelation functions are calculated explicitly using a one-dimensional model for describing the inversion motion of ammonia and its cation, adopting a position-dependent mass for considering the coupling with the stretching mode of the hydrogen atoms in neutral ammonia. This results in a clear physical picture explaining the occurrence of the previously predicted maximum in the ratio of the autocorrelation functions. Furthermore, different initial states and two different ways of incorporating strong-field corrections to the Franck–Condon approximation are briefly discussed.