Commentary for Niddah 6:5

כל שחייב במעשרות מיטמא טומאת אוכלים – for there isn’t a thing that is obligated in tithing that isn’t food (see also Tractate Maaserot, Chapter 1, Mishnah 1).

Whatsoever is subject to tithes is susceptible to food-uncleanness. Only food is subject to tithes and terumah. Therefore, anything that is subject to tithes must be a food.

ויש שמיטמא טומאת אוכלים ואינו חייב במעשרות – as for example meat, and fish and eggs, which do not grow in the ground (note: but it should be pointed out that cattle are tithed – see Tractate Rosh Hashanah, Chapter 1, Mishnah 1).

But there are foods that are to food-uncleanness and not subject to tithes. However, not all food is subject to tithes and terumah, for instance things that don't grow from the ground (see Maasrot 1:1). Therefore, there can be food that is subject to impurity but not liable for tithes and terumah.