[atlas] Actual measurement interval much larger than planned

Dear list,

I encountered some cases where the actual time interval between two neighbouring measurements being much larger than the scheduled value.
Have you witnessed similar cases? I appreciate explanations on underlying reasons.

Here goes an example:

from ripe.atlas.cousteau import AtlasResultsRequest
filters = dict(msm_id=1010, probe_ids=[16981], start=1470055258, stop=1470056217)
is_success, results = AtlasResultsRequest(**filters).create()
if is_success:
    for mes in results:
        print mes.get('timestamp', -1)

The above code requests some built-in ping measurements (240s interval) made by probe 16981 and prints their timestamps. See below for the execution result:

1470055259
1470055499
1470055978
1470056216

Four measurements are retrieved within the given time range.  I noticed that time interval between the 2nd and the 3rd measurement is 479s much larger than the planned value 240s.
How come?

Many thanks for your attention.

Best regards,
wenqin
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