No real explaination, Snape was said to be a master of occlumancy, but so was Voldemort. The best I can figure it that Voldemort was blinded into seeing what he wanted to see, plus Snape’s skill, plus the fact that Snape was driven by a force Voldemort couldn’t comprehend - Snape’s love of Lilly.
'twas beauty that slayed the beast...