Compare the 7 degree polynomial fit of omi Cet data (see attached). The fit is fine but the equation is clearly wrong. Also, the zero point being subtracted from each time parameter is near the mid-point of the date range.
Compare this to the 23 degree polynomial fit of Kepler RR Lyr data, which is correct. The zero point being subtracted from each time parameter is near the start of the date range. The other major difference is that it is quite a short range compared to the omi Cet data; a short filtered single-cycle section was filtered for analysis. What would this look like if the whole FITS file's range was used?