I think the percentage perspective was based on middle income families. Some expenses, like utilities, gas, and even food to an extent, are non-negotiable. My electric bill isn't much different from my wealthy friend's. Food expense can only go down so low, realistically.
If your income is $2100, and you pay $700 rent (lowest 2 bedroom apartment in my area), that leaves $1400 for everything else. Probably doable, depending on the cost of living in the area. Most families would rather pay more rent, and get in a better neighborhood, or look for a small house. Probably $1000-1200. Down to around $1100 after rent. This is getting tight in my area.
If you make $1200, 1/3 would be $400. There is nothing around here but a single room in that price range. If you somehow found an apartment at that price, you would only have $800 for all other expenses. Your food and utility needs would not change much, no matter how frugal you are.
If you make $3300/mo, and you pay $1100 in rent, you have $2200 for other expenses. In this case, obviously you would have more for anything else.
I think the financial advisers who made that suggestion were thinking of folks at the latter level and above. They were suggesting not to spend too much on rent/mortgage with the idea that savings is important. It is of course, but doesn't fit the reality of those of us just getting by. Basic expenses don't change by percentages; they can only be tweaked so far.