It _might_ be 'even order harmonics', though these are generally much lower in magnitude than the odd order harmonics that cause problems on three phase systems.
In a three phase system, any multiple of 3rd harmonic current will be 'in phase', and _add up_ on the neutral even when the fundamental currents balance out. Non-linear loads create odd harmonics. In a non-linear load the current flowing is not proportional to the instantaneous applied voltage, so that the current flow waveform has a different shape than the applied voltage waveform.
In a single phase center tapped system, any multiple of 2nd harmonic current will be 'in phase' and add up on the neutral even when the fundamental currents balance out. In order to generate even order harmonics, a load must be both non-linear and show 'hysteresis' (see http://hyperphysics.phy-astr.gsu.edu/hbase/solids/hyst.html
for an example in magnetic materials). Hysteresis means that the current flow for a particular applied voltage depends upon the _history_ of the current flow, so that the wave shape going up is different from the wave shape going down.
If you can get a scope, look at the shape of the current flow waveform on the phases and on the neutral. If you see 60 Hz on the neutral with balanced loads then something very strange is happening or you have 2 phases of a wye system as iwire suggests. If you see 120 Hz then you have 2nd harmonic. If you see 60 Hz, and you confirm that this is a single phase system (and not a pair of hots tapped from a three phase system), then I would start investigating for something like a neutral to ground fault with one circuit not actually returning to neutral.