Thank you for the advice
I am using Moodle 3.0
The course grade settings are largely set at default. The "Range decimal points" is set to 0.
the "Aggregation position" is set to default, Last.
I have cleared the cache, and restarted the computer.
But the decimal marks are still there.
More importantly when I click "Save and go to next page", it reloads the same quiz page with the same marks. All 7 of the attempts have a number in the appropriate box, some ordinal and some decimal. If I change a mark on an attempt from 4.0000002 (out of 6) to 4, then click "Save and go to next page", the page reloads and the mark reverts to 4.0000002.
It appears that Moodle is expecting a further input from me.
PS in a recent upgrade of Moodle, the error trapping response to "Save and go to next page" changed. Previously, only the attempt with an incorrect or incomplete mark was returned. Now all attempts (good and bad) are returned, so it can be difficult to discover the error. This seems to be the case here. Perhaps one or more of these mark entries is wrong, and I have no way of identifying which one.
This does not seem to be catastrophic as I can close the Moodle page on my computer and restart, carefully avoiding this question. When I look in grade report, the question grades are listed correctly (as ordinal numbers), and the quiz is correctly totaled.