Which food measurement to use?

I've noticed that several of the food packages in my house list serving sizes in both weight (g or oz) and a number of pieces (6 slices of ham or 24 crackers). However, I've found several occasions where these two measurements don't equal each other. For example, the deli meat I used to buy listed a serving as 6 slices, but the weight was consistently equivalent to only 5 slices. Sometimes it's only off by a little bit, which is understandable, but in other cases, it's a fairly significant difference. Does anyone know which of these measurements is the most accurate? For now, I just go with the smallest amount.