Boston's use is remarkably low. I think it's over half of Melbourne's use is industry, so I'd take a wild stab that Boston gets an unusually large part of its wealth from services rather than industry, which would use much less water. That's just guesswork, though.
A curious anecdote - in Melbourne in the '50s and '60s, it was actually against the law to have a private water tank, and this wasn't changed until the '90s, from memory. My mother installed an illegal water tank in the late '80s. It sounds crazy now, but Back In The Day, water tanks weren't well sealed, and Melbourne had a massive mosquito problem. Fast forward a few decades, mosquitos are no longer a problem but drought is, so now you get incentives to install tanks...
Ah, found the difference. That 150 liters/day for Boston is classified as 'residential use', whereas the Melbourne figure is for all uses. The Boston figure doesn't include industrial use.
I had assumed that that comparison graph used the same measure for all cities. Its Santa Fe value matches that described in http://www.santafenm.gov/index.aspx?NID=168 : "The City of Santa Fe computes its per capita water use as the daily average of annual total water diversions from all sources of supply, less bulk water deliveries to the County of Santa Fe and Las Campanas, divided by the estimated customer population served." That's what you were using for Melbourne, but it seems that that's not what the Boston plot was using.
But Santa Fe has little in the way of industry. "Of the 8,500 acre-feet
billed, the single family residential sector used 4,530 acre-feet (53%), the multifamily sector used 878 acre-feet (10%), and the commercial sector used 2,743 acre-feet (32%). Irrigation use accounted for 330 acre-feet (4%)."
At 5408 acre-feet per year, with a service population of 80,000 people, that's about 230 liters per day of residential use per person, which is still more than Boston's residential use. I think that's attributable to how Boston gets much more rain; 41.5"/1050mm vs. Santa Fe's 13.8"/350mm.
How much rain did Melbourne get during those 10 drought years? It looks like around 650mm, so you can see how Melbourne is a somewhat wetter place than Santa Fe. Santa Fe encourages rain barrels, and I can understand how they would also be effective in Melbourne.
(I also don't know how private well usage affects these numbers.)
(Edit: Ooops! Mixed up Melbourne and Sydney - now corrected.)
A curious anecdote - in Melbourne in the '50s and '60s, it was actually against the law to have a private water tank, and this wasn't changed until the '90s, from memory. My mother installed an illegal water tank in the late '80s. It sounds crazy now, but Back In The Day, water tanks weren't well sealed, and Melbourne had a massive mosquito problem. Fast forward a few decades, mosquitos are no longer a problem but drought is, so now you get incentives to install tanks...