Behavior of Natural Abundance and Mass Spec Ratios
You are probably mixing natural abundance (NA) and relative abundance (RA).
In mass spectrometry RA is a more valuable parameter as it can be directly obtained as the $y$-coordinate of a plotted mass spectra: the most abundant ion (isotope) corresponds to the base peak, which is always $100\%.$
In other words, RA reflect isotope ratio, not NA.
For the isotopes of the elements RAs can easily be derived from NAs via normalization; however, the problem of finding RAs of the various isotopic molecular species is a bit less trivial [1].
The following table contains compiled data for NAs [2, p. 1-12] and RAs [3, p. 89] for both elements you've mentioned:
$$
\newcommand{\d}[2]{#1.&amp;\hspace{-1em}#2}
\begin{array}{lllrlrlr}
\hline
Z &amp; \text{Isotope} &amp; &amp; &amp;\text{Mass}/\pu{u} &amp; \text{NA}&amp;\hspace{-1em}/\% &amp; \text{RA}&amp;\hspace{-1em}/\%\\
\hline
16 &amp; \ce{^{32}S} &amp; \ce{[E]}      &amp; \d{31}{9720711744(14)} &amp; \d{94}{99(26)} &amp; \d{100}{000} &amp;\hspace{-1em} \\
   &amp; \ce{^{33}S} &amp; \ce{[E + 1]}  &amp; \d{32}{9714589098(15)} &amp; \d{0}{75(2)}   &amp; \d{0}{789} \\
   &amp; \ce{^{34}S} &amp; \ce{[E + 2]}  &amp; \d{33}{96786700(5)}    &amp; \d{4}{25(24)}  &amp; \d{4}{433} \\
\hline
17 &amp; \ce{^{35}Cl} &amp; \ce{[E]}     &amp; \d{34}{96885268(4)} &amp; \d{75}{76(10)}    &amp; \d{100}{000} \\
   &amp; \ce{^{37}Cl} &amp; \ce{[E + 2]} &amp; \d{36}{96590260(6)} &amp; \d{24}{24(10)}    &amp; \d{32}{399} \\
\hline
\end{array}
$$
References

Margrave, J. L.; Polansky, R. B. Relative Abundance Calculations for Isotopic Molecular Species. J. Chem. Educ. 1962, 39 (7), 335. DOI: 10.1021/ed039p335.
Haynes, W. M.; Lide, D. R.; Bruno, T. J. CRC Handbook of Chemistry and Physics: A Ready-Reference Book of Chemical and Physical Data, 97th ed.; Taylor &amp; Francis Group (CRC Press): Boca Raton, FL, 2016. ISBN 978-1-4987-5429-3.
Gross, J. H. Mass Spectrometry: A Textbook, 3rd ed.; Springer International Publishing: Cham, Switzerland, 2017. ISBN 978-3-319-54397-0.