The Nikkei 300 is, you're linking to the nikkei 225.
The nikkei 300 index is "The Nikkei 300 is a market value-weighted index of the 300 major issues on the first section of the Tokyo Stock Exchange (TSE). The Nikkei 300 uses a weighted average based on market capitalizations of component firms to reflect movements in the overall value of first section stocks. This contrasts with the widely followed Nikkei Stock Average which is an average price and is suitable for monitoring the level of the market and its changes.(1)"
The Nikkei 225 index is "The Nikkei Stock Average is the average price of 225 stocks traded on the first section of the Tokyo Stock Exchange, but it is different from a simple average in that the divisor is adjusted to maintain continuity and reduce the effect of external factors not directly related to the market(2)"
