Datetime inside lambda giving same time for different invocations.

I have a lambda function that writes to dynamodb and inside the lambda, I fetch current time and pass it as well with data as the write time. I have noticed that in the DB, the datetime for the write is same for multiple invocations although they were not together. at least a gap of few minutes was there before each invocation. however, in db, the datetime is the exact including seconds for a few calls that were within a timespan of 15 minutes or so. Can I know why?