Market cap: current rate versus average rate

Schweser book 2 page 219, concept checkers for reading 24: Integration of Financial Statement Analysis Techniques.

Questions 7 and 8.

The current exchange rate is used to calculate Small’s market cap owned by Big.

Why is the current exchange rate used?

Market Cap is the market value of a company’s outstanding shares. The common stock should be valued with historical rate under both temporal and current rate methods.

Anyone knows why? Thanks!