You're right that new games are added to the "Instant Game Collection" each month, but they don't all disappear again at the same time. Some games only stay for the month, but others stay longer.
For example, in October 2013 in Europe, Metal Gear Rising and Remember Me were both added at the 23rd, but Metal Gear Rising disappeared in November and Remember Me stayed until the end of January 2014.
In your case, Brothers was added in March and Bioshock Infinite in January, but they still haven't been removed, while other games like Mero: Last Light or Tomb Raider, that were released around the same time are not available anymore.
Release dates and games differ between North America and the rest of the world. On wikipedia you can find a list of all games that have been released in the "Instant Game Collection", and when they were added and removed, for North America and the PAL region.