Most video codecs use lossy compression, like .jpg; the higher the compression, the worse the quality. You can save the video uncompressed (very large file), choose a codec with configurable compression ratios (lower compression = better quality), or use a lossless codec like Huffyuv. If you don't have Huffyuv, it's available here. In the Huffyuv configuration dialog panel, be sure that the options Always Suggest RGB Format and Enable RGBA are enabled.