Yes, it is, which is why that load must be within the secondary specs. If you want to get really picky about it you match the reflected load current to the primary current, but when you try to do that via the speaker impedance and/or the number of output valves you run into the issue that speaker impedances aren't a fixed value, they vary with frequency and the enclosure design, so you can't really get it perfect. For the most part close enough is good enough so long as you don't have a gross mismatch, like using a 4 ohm tap with a 16 ohm speaker.