In conversational and spoken English, it's common to hear statements like
I only answer phone calls from people I know.
The "correct" form of this would be
I answer phone calls only from people I know.
In rare situations this can lead to ambiguity, but usually only in written English; when speaking, emphasis and intonation clears all doubt about what is 'only'. For instance, someone might say
You only run during the day.
Which of the possible meanings is intended could be clarified by adding more information or by changing the position of 'only':
(1) You only run during the day, but you also walk at night.
(2a) You run only during the day. or
(2b) You run during the day only.
It seems that 'only' moves only leftward, farther away from what it qualifies. A similar thing happens with 'either'.
Is there some sort of explanation for this movement?