Method
The ventral line
The underside runs from the scrotal junction and follows any upward curve, so it is longer than the dorsal line by an amount that varies with anatomy.
Measure along the underside instead of the top and you will almost always get a bigger number. The reason is geometry, not a mistake in the reading itself.
A different starting point
The standard method measures along the dorsal line, the top surface, from where the ruler presses into the pubic bone to the tip. The underside, the ventral surface, effectively starts further back, near the scrotal junction, because that is where the shaft's underside tissue begins in a way that is distinct from the base landmark used on top. A ruler or tape following that line is covering more distance before it even reaches the same tip.
Curve adds more on one side than the other
Most erect shafts carry at least a slight upward curve. On a curved cylinder, the outer edge of the curve is longer than the inner edge - the same reason the outside lane of a running track is longer than the inside one. The top of an upward-curving shaft is the inner edge and the underside is the outer one, so the underside gains extra length from the curve on top of its different starting point. The more pronounced the curve, the larger that gap gets, and it is specific to each person's anatomy rather than a fixed offset you could subtract.
Why this makes the number incomparable
Neither of these effects is an error in technique. An underside reading, done carefully, is a real distance, taken correctly for the line it follows. It is just not the distance the published data report, so it cannot be placed against a percentile built from Veale et al. (2015) or compared to anyone else's dorsal figure without silently overstating the comparison. Two men with identical dorsal-line lengths can produce noticeably different underside figures if one has more curve than the other, which means the ventral number is not even comparable between two people measuring themselves the same way, let alone against a published mean. That is the practical reason to avoid it as a reporting convention, distinct from any question of which line is "correct" in some deeper sense - it simply carries a variable the dorsal line does not. A longer discussion of why the top line is the standard, against the lateral line too, covers the full picture; this note is only about the size of the ventral excess and where it comes from.
If what you actually want out of a number is a subjective read on a photograph rather than a distance, that is a different exercise entirely - the kind Rate Cock or Penis Rater work from, scored by AI Penis or a person at Rate Penis - and none of it is affected by which line a tape happened to follow.