Let's assume that your measurement method is OK.
A general purpose wideband signal generator will tend to break its output range up into several frequency bands, and use a different technology for each band, for output amplifiers and even for the generation method.
Typically, below a hundred (you're not clear what here in your question, it could be bananas, but I'm guessing it's MHz), the generator is using a DDS (though older types use a BFO (beat frequency oscillator) which heterodynes the normal RF frequencies down to low frequency), and the output amplification is provided by a wideband 'op-amp' type or discrete bipolar buffer.
Above 50MHz, there are very nice RF heterojunction transistors that provide high output at remarkably low distortion. Unfortunately these do not work below 50MHz.
The output attenuator, if built from high linearity RF MOS switches, will also have headroom problems below several 10s of MHz.