Phono cartridge left / right output voltage differ

That translates into (almost) exactly a 1.0 decibel imbalance. It'll be quite audible, and some (myself included) will find it extremely annoying. Others are oblivious to channel imbalance until it's more severe.

Unfortunately - until you get into the big money cartridges - many models are spec'd by the manufacturer at no better than a 1.5 dB channel differential. Some models are spec'd at 1.0dB, and once you get into the high end, 0.5dB is more common. So your cart is almost certainly considered normal - especially since it was stated right on the spec sheet!

You could try to mitigate this by playing off other imbalances that may exist downstream. E.g. if you have an imbalance due to asymmetric room acoustics (and you don't mind swapping L/R). Or, you could have a custom 1.0dB attenuator built and place it between the weaker channel's phono -> preamp link. Or you could return/sell that cart and keep looking.

Just to reiterate - Almarg (plus my own posting) is correct - the difference in your cart is *definitely* 1.0 dB, because:
20 * Log_base_10(16/18) = -1.023

^ You get the same magnitude of result, but with a positive sign, if you flip the numerator and denominator. The multiplying factor here is 20, not 10, because we're dealing with voltage proportions, and power (as eventually driven through your speakers) is proportional to the *square* of the voltage (hence the extra factor of 2).

And that's absolutely quite audible, though I've been surprised by how tolerant some folks (even hi-end folks) are about imbalances up to ~ 1.5dB. It's FAR easier to discern a 1dB difference in L/R channel level than it is to notice a 1dB change in level for both channels. In my system, center image would seem shifted by about a foot. I've found myself annoyed by imbalances down to 0.5dB or even less (an almost certainty with vinyl).