There are a number of reasons the advance vote could be up
1) The ongoing trend to get people to vote at the advance polls
2) People leaving for holidays
3) One of the parties is managing to get their vote out.
Here are the advance votes of the last two general elections and the latest by-election
Election Day 1 Day 2 Day 3 Day 4 Total
By-election 2,044 1,395 1,073 1,265 5,777
2013 Election 1,449 1,159 991 1,050 4,649
2009 Election 1,282 859 793 852 3,777
The number of advance voters increases around 1,000 votes in each election. The increase is a consistent one across the days.
If I were to guess, I think the higher turnout is just a reflection of the continued trend of more people advance voting and not any sort of indication of a strong turnout for the by-election. The 2009 and 2013 advance vote results show a small trend in favour of the Liberals over the NDP compared to general voting day.
This higher advance vote likely means that the special ballots will also be higher in the by-election. The special ballots have tended to be slightly stronger for the NDP than the Liberals.
Interestingly, the total number of registered voters as of June 19th was 45,649, on April 23rd (the equivalent date during the general election) there was 44,830 registered voters. This is an increase of 819 voters or 1.83% in less than two months.