One decimal in the table

A published mean reported as, say, 13.1 cm is not claiming the true figure is exactly 13.1 and nothing else. The one decimal place is a statement about how precisely the underlying data support the number, and treating it as exact in either direction is a small but real misreading.

Why one decimal is the right amount

A mean calculated across thousands of individual readings, each with its own measurement error, cannot honestly claim more precision than its inputs support. Reporting to one decimal in centimetres reflects roughly what a careful clinical measurement can resolve - finer than that starts implying a precision the ruler and the observer never actually delivered, and a journal's convention of one decimal is a reasonable, conservative choice given that.

Where it goes wrong downstream

The common misreading is not underestimating a published figure's precision - it is overestimating it. A figure converted from centimetres to inches and reported to two decimal places is the clearest version of this mistake: converting 13.1 cm gives 5.16 inches by arithmetic, but those two decimal places carry no more real precision than the single decimal place the original had, because the conversion cannot manufacture information that was not there. A number that looks more precise after conversion is not more precise - it is the same precision, dressed up.

Rounding is not hiding anything sinister

This is not a criticism of the papers. One decimal is the appropriate resolution given how the data were gathered, and every reported mean in the pooled clinical literature follows the same convention for the same reason. The point is narrower: read a published mean as good to about the stated decimal, not finer, and be suspicious of any figure - your own included - that claims more precision than its measurement process could have produced.

The same logic applies to your own readings

If three readings taken by hand spread by half a centimetre, reporting the result to a hundredth of a centimetre is the same category of error as reporting a converted inch figure to two decimals - the extra digits are decoration rather than information, borrowed from arithmetic rather than earned by measurement.

A published mean is a different kind of number from a rating. Rate Cock and a scoring platform like Penis Rater report a score, not a mean with an implied precision from pooled measurements, so this specific caution about decimal places does not carry over to a photo-based judgement of the kind Rate Penis or AI Penis produce - a score's precision is a different question, worth asking of those sites on their own terms rather than by analogy to a published table.