Two teams, one park, different park factors.

From 1903 through 1975 two teams (one AL, one NL) have shared the same ball park in a season 65 times.  None since then.  Only the two seasons (1974-1975) the Yankees and Mets shared Shea Stadium did the AL team use the designated hitter (DH).

In New York the Yankees and Giants shared the Polo Grounds 1913-1922.

In St. Louis Cardinals and Browns shared Sportsman's Park 1921-1953.  Then the Browns morphed into beautiful birds, the Baltimore Orioles.

In Philadelphia Athletics and Phillies (Blue Jays some seasons) shared Shibe Park 1939-1954.  In 1955 the Athletics moved to Kansas City.  Shibe Park was renamed Connie Mack Stadium 1953-1976 after the Athletics owner.

In Los Angeles Dodgers and Angels shared Dodger Stadium 1962-1965.

So how come the park factor is different for one team almost every season?  Because baseball is played in non-uniform playing areas a park factor is needed to try to equate stats.  There are actually two park factors, one for batters (BPF), one for pitchers (PPF).

BBF is equal in only 11 of the 65 shared seasons:

yearID	park	TeamAL	TeamNL	BPFal
1921	Polo Grounds IV	New York Yankees	New York Giants	102
1922	Polo Grounds IV	New York Yankees	New York Giants	102
1928	Sportsman's Park IV	St. Louis Browns	St. Louis Cardinals	103
1933	Sportsman's Park IV	St. Louis Browns	St. Louis Cardinals	106
1934	Sportsman's Park IV	St. Louis Browns	St. Louis Cardinals	107
1935	Sportsman's Park IV	St. Louis Browns	St. Louis Cardinals	104
1949	Shibe Park	Philadelphia Athletics	Philadelphia Phillies	97
1950	Shibe Park	Philadelphia Athletics	Philadelphia Phillies	98
1950	Sportsman's Park IV	St. Louis Browns	St. Louis Cardinals	104
1954	Connie Mack Stadium	Philadelphia Athletics	Philadelphia Phillies	100
1974	Shea Stadium	New York Yankees	New York Mets	99

In St. Louis when the teams had the same BPF three consecutive years it was different each year: 106, 107, 104.  Same thing in Philadelphia for two consecutive years: 97, 98.  Oddly the Yanks with the DH and Mets are one of the 11 seasons when the teams had the same BPF.

PPF same:

yearID	park	TeamAL	TeamNL	PPFnl
1921	Polo Grounds IV	New York Yankees	New York Giants	98
1922	Polo Grounds IV	New York Yankees	New York Giants	98
1938	Sportsman's Park IV	St. Louis Browns	St. Louis Cardinals	104
1939	Sportsman's Park IV	St. Louis Browns	St. Louis Cardinals	105
1940	Shibe Park	Philadelphia Athletics	Philadelphia Phillies	101
1941	Shibe Park	Philadelphia Athletics	Philadelphia Phillies	101
1941	Sportsman's Park IV	St. Louis Browns	St. Louis Cardinals	104
1946	Shibe Park	Philadelphia Athletics	Philadelphia Phillies	100

Two instances when both BPF and PPF were the same:

yearID	park	TeamAL	TeamNL	BPFal	PPFnl
1921	Polo Grounds IV	New York Yankees	New York Giants	102	98
1922	Polo Grounds IV	New York Yankees	New York Giants	102	98

Park factor is used to compute OPS+ and ERA+, two bedrocks of current conventional wisdom.  I realize that parks change from one season to the next, even the configuration of the same park changes, and that there are different parks in the two leagues but this indicates how little we know about this important stat other than the vague notion that it somehow mysteriously takes the different and non-uniform playing areas into account in order to enable us to compare Albert Pujols and Lou Gehrig.

We really should have a much better understanding of park factor.  Wins Above Replacement (WAR).  All that stuff.