The return value of function getXpathCount is wrong

Hi, I use the function getXpathCount("//table[contains(@class, '"myTable')"), but the return value is always 0.

And then I use the function isElementPresent("//table[contains(@class, '"myTable')"), return true. So who can help me to check why getXpathCount doesn't work?

By the way, the table is in a iframe.

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