I noticed this :
If you specify 2s as a manual step, then the solver will use it. But in the same time if you specify a saving every 15s for instance, after 7 steps the next one cannot be 2s anymore. So the solver overrides your value to 1s so that the saving occurs at 15s exactly. Please check if this is not what happens here...
Thanks, I looked at the plot data in the excel spreadsheet and it appears this is when the time step adjust.