Not a shock, Heady. The 55s (nominal manufacturer volume) measure by outside dimensions at ~56 gallons, but actually hold less than 50. Measured volumes at setup for me is about 41 gallons. My 29 actually holds 21 gallons. (I use fairly deep substrates for planted tanks).
But all the rules of thumb are based on the manufacturer's nominal volume, so how you stock is personal choice. I stock lightly (less than 1" - using standard length- of adult fish per gallon of nominal tank water). I do that because is sooo much easier to maintain good water parameters a bit understocked than overstocked.
Part of stocking also depends on the particular fish you keep - the rules of thumb really refer to small slender-bodied fish less than 3" long - meaning most small schooling fish like Tetras, small Rasboras, etc. Beyond that you have to judge by the the fish themselves - Barbs tend to be hyper, so I allow them more space. Rainbows are very active also, so get more space. My puffers are usually kept alone, but are as messy as Goldfish and Oscars, so get loads of space.
With current filtration you can keep much more than the rules of thumb "allow", but the maintenance and uninterrupted power become critical and even more important. I like to give myself a bit more margin.
