Why are superscripts used instead of subscripts in this example?

A snippet from Marcus du Sautoy's The Number Mysteries, Chapter 3, in the section called HOW GOOD ARE YOU AT RANDOMNESS is shown below. My question is, why does the author use superscripts instead of subscripts here?

-
No other reason than personal preference. – Austin Mohr Sep 26 '11 at 3:44
It's poor notation especially here though, with $2^N$ used in one line and $g^N$ in the next, with different meanings for the $^N$. – ShreevatsaR Sep 26 '11 at 3:45
Maybe nobody bothered to proofread. – André Nicolas Sep 26 '11 at 4:36
@Andre and J.M.'s suggestions are surprisingly plausible. I recently sent in a proof-revision for a paper that I wrote where the journal's typesetters managed to systematically convert every instance of $T_pM$ (the tangent space of the manifold $M$ at the point $p$) to look like $T_{pM}$. A lot of crazy things happen at the type-setting stage for mathematics books and papers. – Willie Wong Sep 28 '11 at 9:28
Before the widespread use of TeX, and the possibility that the author does the typesetting, it would be quite common that the typesetter did not know mathematics. – GEdgar Sep 28 '11 at 12:30
{"extraction_info": {"found_math": true, "script_math_tex": 0, "script_math_asciimath": 0, "math_annotations": 0, "math_alttext": 0, "mathml": 0, "mathjax_tag": 0, "mathjax_inline_tex": 1, "mathjax_display_tex": 0, "mathjax_asciimath": 0, "img_math": 0, "codecogs_latex": 0, "wp_latex": 0, "mimetex.cgi": 0, "/images/math/codecogs": 0, "mathtex.cgi": 0, "katex": 0, "math-container": 0, "wp-katex-eq": 0, "align": 0, "equation": 0, "x-ck12": 0, "texerror": 0, "math_score": 0.6209556460380554, "perplexity": 570.5139604390885}, "config": {"markdown_headings": false, "markdown_code": true, "boilerplate_config": {"ratio_threshold": 0.18, "absolute_threshold": 10, "end_threshold": 15, "enable": true}, "remove_buttons": true, "remove_image_figures": true, "remove_link_clusters": true, "table_config": {"min_rows": 2, "min_cols": 3, "format": "plain"}, "remove_chinese": true, "remove_edit_buttons": true, "extract_latex": true}, "warc_path": "s3://commoncrawl/crawl-data/CC-MAIN-2016-07/segments/1454701167113.3/warc/CC-MAIN-20160205193927-00119-ip-10-236-182-209.ec2.internal.warc.gz"}