Like the old library the current one has the problem that on some aspect ratios of the map the click handler on a cluster does not zoom in.
googlearchive/js-marker-clusterer#135
The is reproduceable in the examples. When you define a small height for the canvas like 200 pixels or so. In my page it does not work for a width of 850 px and a height of 250 pixels. When I use a height of 500 pixels everything is fine.