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:
$$
\newcommand{\d}[2]{#1.&\hspace{-1em}#2}
\begin{array}{lllrlrlr}
\hline
Z & \text{Isotope} & & &\text{Mass}/\pu{u} & \text{NA}&\hspace{-1em}/\% & \text{RA}&\hspace{-1em}/\%\\
\hline
16 & \ce{^{32}S} & \ce{[E]} & \d{31}{9720711744(14)} & \d{94}{99(26)} & \d{100}{000} &\hspace{-1em} \\
& \ce{^{33}S} & \ce{[E + 1]} & \d{32}{9714589098(15)} & \d{0}{75(2)} & \d{0}{789} \\
& \ce{^{34}S} & \ce{[E + 2]} & \d{33}{96786700(5)} & \d{4}{25(24)} & \d{4}{433} \\
\hline
17 & \ce{^{35}Cl} & \ce{[E]} & \d{34}{96885268(4)} & \d{75}{76(10)} & \d{100}{000} \\
& \ce{^{37}Cl} & \ce{[E + 2]} & \d{36}{96590260(6)} & \d{24}{24(10)} & \d{32}{399} \\
\hline
\end{array}
$$
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 & 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.