This does not resolve the conflict, but traces its origin.
One of the first links that appears from your Google query is for
Internet Live Stats that also presents the 84.2% number and what seems to be a justification by giving the total population and the number of internet users. That source cites "Elaboration of data by International Telecommunication Union (ITU), World Bank, and United Nations Population Division." (bolding mine) So it is odd that it disagrees with the World Bank number.
There is something suspicious about the Internet Live Stats data. Using the 2013 data to match the Google chart, they give the basis for their percentage: the number of internet users (267,028,444) and the number of people in the US (317,135,919). First of all, Note that these figure are reported with 9-digit accuracy. This figure could only have been accurate for a single day in 2013. Acording to the
US Census Bureau US population was:
6 Nov 2013 317,135,156
7 Nov 2013 317,141,277
Similarly, the number of internet users changes rapidly, so 9-digit reporting is odd.
However, note that
267028444 / 317135919 = 0.84200000063695091. It seems a bit unlikely that it works out so that the ratio is a one-decimal place percentage to 9 digit accuracy. The reported percentage was surely not calculated from these numbers. Rather, at least one of the numbers was calculated from the percentage.
I see that the World Bank cites "International Telecommunication Union, World Telecommunication/ICT Development Report and database, and World Bank estimates"
so both of them are using ITU data. Curiouser and curiouser.
So there is nothing left to do but look at the ITU. I could only find the proportion data at the ITU (not the population numbers used to calculate the percentage), but this page at the
ITU offers a couple of useful spreadsheets. There is a link for a spreadsheet call
Percentage of Individuals using the Internet
It says that the US percentage of individual users of the internet was 74.45 in 2015 (71.40% in 2013). The same page also offers
Core indicators on access to and use of ICT by households and individuals
This second spreadsheet says that the percentage was 74.6% in 2015. Those two seem to point to the lower figure. BUT there are some links at the lower right hand side of the page, one of which is for the
Google Data Explorer
which offers "Explore and visualize key ICT indicators with Google Public Data Explorer." Presumably, this is Google analytic tools applied to ITU data. Using that tool, you can select "Percentage of Individuals using the Internet" and "United States". The resulting chart says that the percentage was 84.2% in 2013 (the latest date on the chart).
So the inconsistency seems to originate from within the ITU.
Disclaimer: This is what was available on the mentioned sites on 15 Jan 2017.