Different peak and mininum discharge times for Newfoundland rivers
All of the following parameters would affect seasonal maximum and minimum flow times. Differences between the river basins:


Size of drainage area
Average relief of topography within the drainage area
Amount of groundwater entering the river (near max) 
Amount of water leaking into the local water table (near min), 
Underlying differences in geology of the river basins
Water usage from each river
localized weather patterns 
Sun exposure (for snow melt)


Careful and detailed geological and hydro-geological study of each rover basin would need to be performed to determine exact cause of differences in timing of max and min flow times.