The regulator sets the voltage with the resistors 237 and 1000 ohms. These are +/-1% resistors, and the chip's internal reference has a similar variation. And the series 0.51-ohm/3-watt resistors near the sockets are probably 5% (i.e.1% of the total voltage).
Plus the accuracy of your meter.
Plus, the variation in the resistance of wire and soldered connections.
You may just have an unlucky combination of those variations.
In any case, those 0.51-ohm resistors can be adjusted. To increase the voltage, parallel one or both with a suitable resistor. In most cases, including yours, it's close enough to not worry about.
The exception is with EML tubes. The manufacturer specifies a very tight +/-4% tolerance on filament voltage. That's 4.8 to 5.2 volts. If you are using them you'd want to increase that 4.74 to match the 4.86. At that level of precision it's best to measure the voltage at the physical tube pins rather than at the socket terminals. Fussy, but you only have to do it once!
