You might remember a certain Spain - Russia played twice at EURO 2008. It happened because teams in groups A and B were separated from groups C and B until the final in order to equalize the number of rest days.
This was UEFA's explanation at that time:
"The reason for the format change this year is to equalise the rest periods during the knockout stage. Complaints were made in previous competitions that some teams gained an unfair advantage of up to a day and a half’s more rest between games as the tournament progressed."
For 2012 however, UEFA have returned to the old format, no longer separating the teams as in 2008.
See the official fixture list on UEFA.com.