With the fuel mixture introduced into the crank case in the manner specific to this engine, more than meets the eye happens. If you take a close look at many other (more expensive and likely better developped) engines of the same configuration, you will see that the "injection cage" which is the funnel or trompet or what you may call it, that guides the fuel mixture from the carburetor into the crank case, is angled towards one side. When the crank is spinning and drawing fuel into the case, the direction of the crank and rods and rod bearing has an effect on where the majority of the mixture is contained in the case, which can very well have an effect on the distribution of mixture to the two cylinders.
Anyway, to make a long storey short, the depicted engine may very well have one side running richer than the other. Likely the left side (when viewed from behind) is the richer side.