I can’t give you a direct answer to your query but might give some food for thought.
Firstly, are you confusing the actual polling of the tag by the ModbusTCP device driver, with a scan class setting, and the history stored on the tag? If you have a tag with a scan class of 1s, and a ‘default’ (not ‘default historical’) history option enabled on that tag - well, you can only set the history option once, so to have it at both 1 second and 10 seconds you’d need two separate tags, both pointed at the same Modbus register, with two different history settings.
I’ve read a bit about what goes on behind the scenes, a quick search for anything by @k.herron or @p.turmel is always good reading (there are others too of course 
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e.g. Using runScript, any effects on Heap Memory - #6 by Kevin.Herron
I feel the answer you may get to this is, there is some level of asynchronous nature, so to guarantee interval specific logging, use transaction groups not historian.