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We check the stability of the ABMP16 fit with respect to modifications ofquark PDFs suggested in the recent literature: the strange sea enhancement anda positive non-vanishing $d/u$ ratio at $x \to 1$. These possibilities areexamined using test versions of the ABMP16 PDF fit which demonstrate no need ofthose changes. Furthermore, we localize peculiar features in other analyseswhich are responsible for a different behaviour of the PDFs obtained. Thestrange sea enhancement can be explained by a choice of the PDF shapes beingnot flexible enough and therefore leading to an over-suppressed $d$-quarkdistribution. This suppression has to be compensated by a corresponding rise bythe $s$-quark distribution. As a result, an unusually large strange seasuppression factor is obtained. The non-vanishing value of $d/u\vert _{x=1}$becomes consistent with zero in case the higher-order QCD corrections to theD{\O} $W$-asymmetry data, which drive its value, are taken into account.Furthermore, the related $e$-asymmetry sample prefers a slightly negative valueof $d/u\vert _{x=1}$, although it is consistent with zero. These clarificationssupport confidence in the PDF shapes used in the ABMP16 analyses.

Strange and non-strange distributions from the collider data