The number of presidential vetoes varies from year to year partly because there are different times of presidency when there are partisan or bi-partisan governing. So president may get a partisan control in the congress when his party dominates in both chambers, bi-partisan control is the time when president has support dominance of opposition party in both House of Representatives and Senate and bi-partisan control when opposition party dominates in either chambers. Switch form two types of governing can occur every 2 years period in Mid term elections. George W. Bush has the smallest record of presidential vetoes because out of 8 years of his ruling (2 terms) 6 were under partisan control of Republican party. As result of dominance of his own party in congress he had only 12 vetoes (11 regular + 1 pocket) comparing to 37 vetoes under Bill Clinton. He made relatively high number of vetoes because out of 8 years of his presidency 6 years the congress was under control of opposition Republican party. Ideological incompatibility with Republican dominated congress has lead him to place majority of 37 vetoes.
Variation of numbers vetoes can be caused by public opinion change. Public opinion support to either congress or president can have a major impact on number of presidential vetoes. High percentage of support from citizens to a president will make president more active as people trust him and expect him to produce more laws. So the number of presidential vetoes may rise, especially in bi-partisan control because public opinion swings to a president and show that he must take an action to dominate in the government. However, if opinion polls suggest that the president has low support, he may reduce the number of vetoes by which giving domination to congress. In 2006 George Bush's opinion polls showed that his popularity was very poor. After mid term election where Democrats got majority