No Balanced Circuitry

It means the the input/outputs are balanced but that the circuit topology is not fully differential, which would indicate a positive and negative gain stage for each channel. Twice the circuitry which is why most manufacturers don't do it.

Hi Spin,

So does it mean that there is no sound significant sound different?

Very common for manufacturers to employ balanced connections at the inputs or outputs of unbalanced circuitry.
Especially seen in pro gear, use of IC or transformer balancing (vs. truly differential balancing: doubling of circuitry and the inherent problems of timing two (hopefully identical)180 degree out of phase signals through a circuit without timing, phase or distortion issues.
When transformer or IC balanced, the only sonic benefit occurs when long IC runs are needed, otherwise stick with single-ended.

Twice the circuitry which is why most manufacturers don't do it.

This statement is incorrect. It does not take twice as much circuitry to go differential!

Clarrie, the answer to your question is probably yes. If the circuit is not internally balanced, the chances are very good that the connectors are just there for convenience.

Does it mean that the XLR output are not fully balanced....???

"Fully balanced" is a term that applies to amplifier construction and not the input or output.

Input can be balanced (in non fully balanced amp) by either Transformer or Instrumentation Amplifier. To make output balanced requires additional inverting stage (gain -1). XLR cables are less susceptible to noise, have locks and hidden (female) input pins. Other than greater noise immunity balanced cables should be less transparent than RCA cables because of this additional input and output stage.