This is a caveat to the Laplace's tidal equation fit to the QBO data. Although the seasonally aliased Draconic period works very well, a shifted value from 27.212 days to 27.209 provides an arguably better fit. Although not noticeable to the eye, the correlation coefficient does increase by almost 0.01 with the slightly shorter period. Compare upper vs lower curves below:
This is all about keeping a coherent phase across over 60 years of QBIO data. After 54 years or 650 Draconic-monthly periods, the difference between a seasonally aliased 27.212 and 27.209 will become apparent as a gradual phase shift, as shown below.
The phase buildup is about a tenth of a period. The issue is whether this is due to fitting slop as the multiple linear regression tries to account for possible [jitter](https://en.wikipedia.org/wiki/Jitter) noise or whether this is something real in the physical process. The latter may occur if some other forcing alignments in sync with the lower period value exist. The model is likely within some uncertainty margins, but I don't have a good handle on how to quantify the margin itself.
No matter how well a model works, there is always room for creeping doubt. If you don't have this in your own mind, someone else will certainly express it.