Also, the deviation in the last elections from the mathematically expected was about 4%...
But the end results was 51% vs 49% meaning that if the result went as expected, the 49% candidate would have won (with 53%).
So, in some situations is not that hard to cheat.
EDIT: also even when there is blatant cheating going on, nothing much can be done about it...
Example, during elections for a minor office in one of the smallest Brazillian states (Acre), the number of invalid votes, intentionally blank votes, and voters that didn't show was exact the same, and the total number of votes was higher than the number of voters.
The losing candidate sued, the government argued that "the computer counts, and thus a recount is not a valid request", and FINED the guy that sued.
And the whole argument stopped there.