There are a three reasons this could be happening:
1) The problem could be caused by overlapping text or images. Standard HTML does not support overlapping items, so for each overlapping item an image is created. Thus there will be subtle differences in the display between these generated images and normal plain text.
2) You have used 'non standard' fonts in your design, and are viewing your site on a computer that does not have these fonts installed.
3) The web browser is not capable of displaying the font in the size you have specified. If you build your site as standard HTML, fonts will only display in one of the following point sizes: 8, 10, 12, 14, 18, 24, 36. If you pick a different size, it will be changed to one of the above.
1) Ensure that your text items do not overlap each other, and do not overlap any images.
It is important that no part of any text item - including its surrounding rectangle - overlaps anything else. When you select an item, the handles that appear around it indicate the size of this rectangle. Alternately, you can build your site as Dynamic HTML, which does support overlapping items.
2) It is recommended that the fonts 'Arial', 'Courier New' or 'Times New Roman' be used, as these are the most commonly available fonts.
3) You can build your site as Dynamic HTML, which supports all font sizes.