Why does our corps supplies and tech spare parts keep lowering?

People in your corps that are R4 or higher (e.g. Your leader/general) can use these supplies and spare parts to plant down corp gathering depots, peace-keeping weapons (that create an aegis around your tiles), or can be used to upgrade corp research or corp buildings (e.g. corp command).

If you're the leader, chances are someone is using your corp supplies without telling you~

Another case is that an enemy has destroyed your corp command and has thus stolen supplies from you~