Shouldn't it be ANALYSES (plural) instead of ANALYSIS (singular) here, and why?

I have asked this before, but I didn't get a satisfacting response; moreover, I have asked this at several occasions and I had all sorts of answers, but no one told me why so I could understand. ANALYSIS and ANALYSES both have the same meaning as defined by this dictionary entry: "(countable, context, of a thing, concept, theory, etc.) The process of dismantling or separating into constituent elements in order to study the nature, function, or meaning; the result of this process." The word "analysis" is countable; therefore, ANALYSIS and ANALYSES must refer to different things.

Moreover, it may be difficult to separate the effects
of salt from other nutrients that may contribute to
stomach cancer risk. The absence of adjustment for
confounding factors (such as age, sex, smoking and
dietary habit) can hamper the statistical estimation
causing over- or underestimation of the real association
between salt or salted food and stomach cancer. In
Tables 1 and 2, few studies have controlled for dietary
factors in their ANALYSES of salt consumption, which
makes it difficult to compare the different studies
according to the dietary variables adjusted in the ANALYSIS.
However, the study results that were adjusted by a wide
range of potentially confounding variables, such as age,
sex, H pylori infection, atrophic gastritis, medical history
of peptic ulcer, family history of cancer, body mass
index, diabetes, total cholesterol, physical activity, alcohol
intake, smoking habit and other dietary factors[68],
showed no difference from the crude results. Studies
with adjustment for some or most of the above potential
confounding factors[66,68,72,73] showed no systematically
apparent differences from the studies with adjustment
for a few or several confounders.