Older versions of Netscape Navigator seems to convert pixel-based frame dimensions to whole percentages, and to use those percentage-based dimensions when laying out the frames. Thus, frames with pixel-based dimensions will be rendered with a slightly different size than that specified in the frameset document. The rounding error will vary depending on the exact size of the browser window.
Furthermore, Navigator seems to store the percentage-based dimensions internally, rather than the original pixel-based dimensions. Thus, when a window is resized, the frames are redrawn based on the new window size and the old percentage-based dimensions.
There is no way to prevent this behavior. To accommodate it, you should design your site to adapt to variations in the frame dimensions. This is another situation where it is a good idea to accommodate variations in the browser's presentation.