The problem with Soundscan in the 1990s is that when it began in June of 1991, it only tracked 40% of all total sales at retail stores. By the end of 1993 this increased to about 75%. By 1996, it was up to 85%, and since then it has eventually come to track over 95% of all sales at retail.
The second thing is the proliferation of record clubs like BMG in the early and mid 1990s who sales were never tracked by Soundscan. BMG sold nearly 2 million copies of the Joshua Tree in the 1990s, but not a single copy was tracked by soundscan.
Bottom line, U2 sales since 1991 Soundscan era in the USA are higher than what Soundscan has tracked, but its unknown how much higher they are.
I estimate that U2 has sold a minimum of 32 million albums(of their entire catalog) in the USA since June of 1991 when Soundscan began.