Actually US.UKfan there have been 538 since 1964, not 1972.
After the 1910 census they decided to fix the congress at 435 effectively fixing the electoral college at 531 (there were 48 states with two senators each: 48*2=96, 96+435=531). There were 531 votes in every election 1912-1956. Then in 1959 Alaska and Hawaii became states: The senate increased to 100 members and the House increased to 437 since Alaska and Hawaii both had to have one representative. Thus there was a total of 537 for the 1960 election. When the results of the 1960 census came in after the election, the number of representatives was changed back to 435 (it still remains at 435 to this day). However around the same time a Constitutional ammendment was passed giving DC the total number of electoral votes it would be entitled to if it were a state, namely 3. By the time the ammendment was passed the electoral college had shrunken to 535 due to the reaportionment after the 1960 census, the addition of DC's 3 votes brought it up to 538 where it stands today.