First of all, “matched” refers to set of power amp tubes. Preamp tubes are not sold as matched sets, nor are matched sets required.
In the early days of tubes amps nobody really worried about matched sets of power amp tubes, and many manufacturers to this day still do not. I have even heard it theorized that part of the old Fender "sound" was due to the fact that they did not care about matched tubes. They just plugged in the tubes, dialed in a recommended bias voltage, and that was it.
Tubes will test differently from day to day because of fluctuations in the power supply and "drift" in the tube over time as it ages. The first 100 hours of playing, and especially the first 24 to 48 hours, may change matching significantly enough to render it a useless consideration.
Even "burned in" tubes can and will drift over time. Any supplier who tells you that "burned in tubes will not drift" is either lying to you, or does not know what they are talking about.
For that matter, pick up one tube of your new “matched” set. Shake it real hard for a few seconds. Guess what? You no longer have a matched set. Vibration and heat have a major affect on tubes.
Saying that matched tubes are absolutely necessary is a bit of Mojo in itself. This issue is not as big a deal as a lot of people make it out to be. For this to be a big deal, you have to assume symmetrical power supply, and zero component tolerance. Still, it is a good idea to have the tubes matched as closely as possible. It sounds better, tubes may last a little longer, and hum and noise will sometimes be reduced.
Most power amp tubes are sold in matched sets anyway. It bugs the crap out of me though, to see sellers charging extra for "premium matching" or some other such nonsense. It's really just a big ripoff in most cases. For most amps, a plate current match within 20% is acceptable, and 10% is excellent. Matching any closer than that is not necessary in my opinion.