First thing to note is that you shouldn’t be looking to get maximum fares. They don’t count towards capping and may leave you open to a penalty fare or prosecution.
That said, upto £6 is just for off-peak fares, it’s £8.90 for peak fares. Amersham to Baker Street is well within those figures. There are certain situations where you might not be charged the full maximum fare, particularly if you have a travelcard on your Oyster card. It doesn’t really vary from station to station though.
And they’ll all be increasing next month in line with the new fares.