under the assumption that week-week changes in normalized stats would be swamped by my tests This is an enormously problematic assumption, which you can verify

under the assumption that week-week changes in normalized stats would be swamped by my tests This is an enormously problematic assumption, which you can verify by either looking at the week-to-week stats for a period prior to you joining the company, or (for a far more fun demonstration) doing historical testing of the brand of toothpaste you use for the next 6 weeks. Swap from Colgate to $NAME_ANOTHER_BRAND, note the improvement, conclude that website visitors pay an awful lot of attention to the webmaster's toothpaste choices.