Ethereum addresses are case insensitive, and addresses such as 0xABCDEF..., 0xabcdef... or 0xAbCDeF... are same address. On Gastracker addresses are displayed using a special format to avoid mistakes and typos, and make it easier to distinguish similar addresses. By using a special formula some of the letters are in upper case some on lower case.
This a part of Ethereum specification, specifically it's EIP-55 and it's officially recommended way to display addresses on Ethereum ETC/ETH networks.
This format is used for displaying purposes only and it doesn't affect actual case-insensitive address