[JohnM]

The display is accurate at all settings, but it has lower frequency resolution than an equivalent RTA below the frequency shown in the mode line. The RTA octave fraction and FFT length setting only change the frequency resolution of the display, not its accuracy.

Signals processed by FFT have a frequency resolution equal to the sample rate divided by the FFT length, for example a soundcard sampling at 48kHz and an FFT length of 65536 (64k) gives a frequency resolution of 48000/65536 = 0.732Hz. This is often caled the "bin" width of the FFT. An RTA has a resolution that varies with frequency as it is a fraction of an octave, i.e. a fraction of a doubling in frequency. The octave between 1kHz and 2kHz has 1000Hz whereas the octave between 10Hz and 20Hz has only 10Hz, so the width in Hz of 1/12th octave (for example) is much less at 10Hz than it is at 1kHz. The indication on the mode line tells you the frequency below which the resolution is determined by the FFT length rather than the RTA octave fraction, above that frequency the underlying FFT resolution is higher than the RTA setting requires, so as many FFT bins as correspond to the RTA octave fraction at each RTA frequency are added together to produce the RTA value. Below that frequency the individual FFT bins are used directly, scaled appropriately to show the correct level.