No way.
As with other physiological determinants of performance (e.g., VO2max), maximal metabolic steady state (for which FTP is a surrogate) is subject to both day-to-day variability as well as longer-term changes due to, e.g., training. Nonetheless, as with VO2max, such variability is quite small, i.e., only a couple percent.* You should therefore be able to readily determine your FTP to the nearest 5 W. If you can’t/aren’t at least that confident in your estimate, you should revisit your test protocol and/or question your equipment.
*This is partially where people go wrong in holding up lactate testing as the “gold standard”, as in fact it is more, not less, variable than just measuring performance.