I'd also add that expressing ounces in a decimal format usually requires some rounding to avoid cluteringly long weights. One ounce is 0.0625 pounds. Is it easier to read a recipe calling for one ounce that lists thirteen ounces as 0.83 lb. or 0.8125 lb.? Rounding makes it an easier read.
Additionally, when using beersmith (and probably other brewing software) the software doesn't make it easy to input by the ounce and rounds a little over or under to get close. I usually have to enter an ounce as 0.07lb. to get close to accurate in beersmith. I'm lazy and continue to use those rounded numbers when I post up recipes. 0.83 lb. in beersmith is 13.3 ounces. That could easily be the explanation.
I strongly doubt anybody is measuring out 0.28 ounces for a porter. It's either a rounding issue in the recipe or scaling down a pro recipe.
You can just type 1 oz, for example, in the amount field and Beersmith will automatically convert it to whatever scale you're displaying. I do that for hops all the time, instead of calculating 28.35 grams for each ounce myself, I just type 2 oz and it does the rest. As others have stated you can also adjust the units that Beersmith displays to whatever you like. I have grains set to display lbs and oz and hops in grams.
You can also set the number of decimal points units will display to. If you prefer something more accurate you can set that. I set mine to match what my scales display.
Go into Tools > Options > Units to adjust these settings to display how you prefer. You can adjust these settings for a variety of different items.