Because Poorva Express via Gaya goes through Varanasi, so extra time taken by Poorva Express (via Patna) is compensated with extra time taken by Poorva Express (via Gaya) by going through Varanasi. Both of the trains follow same timings from Allahabad onwards.
Even Howrah Rajdhani (via Patna) takes more time than Howrah Rajdhani (via Gaya) as both of them follow direct route unlike Poorva Express.