Remember, these voltages are +/- 10% for the stock configuration only.

Modded circuits (including the use of different rectifier, output, or preamp tubes) may cause your amp to show different voltages.

So, why do things turn out different than one might expect - if you are looking at a voltage chart?

1. Variability of the line voltage at your locale.
2. Variation from power transformer to power transformer - if you use the wrong taps, even larger variation.
3. Rectifier variation - especially if you are using a real tube rectifier, worn out versus new, and the possibility of a mislabeled tube.
4. Power tube conduction at idle. Tubes are not exactly precision devices, matched pairs are not going to bias up like the next matched pair. Understanding the effect of the screen voltage on the bias is important - if you change the screen voltage, the conduction changes as does the bias.
5. Resistors and chokes in the power supply affect the downstream voltage drops. This is your chance to learn Ohms Law!
6. Pre-amp tubes have bias variations from each half of each tube and tube to tube. This will change your plate and cathode voltages on these tubes. Substituting a different tube - 12AY7 for 12AX7 for example - will drastically change those voltages also. While different tubes can work, you really need to change the power supply and the plate / cathode resistors to optimize for that tube.
7. Modifications you have read about or come up with yourself can drastically changes things - for the better or worse.