Isochrone results vary depending on the size of buckets

I have asked about isochrones & buckets also before here, but I the keep on puzzling me again.

If I create an isochrone with a time limit of 7200 seconds and 4 buckets, the smallest bucket (i.e. polygon) should equal to 1800 seconds. But if I create a isochrone with a single bucket with a time limit of 1800 seconds from the same point, the difference is quite significant especially outside the graph (road network). See the both results here. The smaller one is the one with a single bucket:

If the results are used e.g. for analyzing how many people (who obviously are on those areas just outside of the network) can reach a certain point, the results vary dramatically. Why the results are so different and which method should be used for analysis?

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