difference number of pageviews between previoiusPagePath and nextPagePath

[phar]

I tried to figure out some numbers of navigation summary with the
api.

I read this discussion.
http://groups.google.com/group/google-analytics-data-export-api/browse_thread/thread/10f980c8e30fd10a/df25be2657671943?lnk=gst&q=How+do+we+use+Navigation+Dimensions#df25be2657671943

I tested it in my data.
But I do'nt understand why the two number of pageviews are different.

1. dimension : ga:previousPagePath, metrics:ga:pageviews,
filters:nextPagepath=~^/hoge$
the resulting pageview is, say, 60.

2. dimension : ga:nextPagePath, metrics:ga:pageviews,
filters:previousPagepath=~^/hoge$
the resulting pageview is, say, 50.

* Report UI tells the number of the pageviews is 60, which is
appropriately equal to previoiusPagePath.

I think the two pageview numbers should be equal, but not.

Could anyone explain me why the number of pageviews of nextPathpath is
less than the other.