One potential reason could be that he actually isn't. The AI tends not to go for super expensive technologies, so alternatively if your literacy rates are similar, it could be that Siam have a lot of simple technologies while you're the one beelining expensive techs. You would get the expensive techs at the same rate Siam would get the cheaper techs, but literacy rate is just a measure of how many technologies you have, so if you get expensive techs, your literacy rate remains artificially low. Once you decide you need to get the cheaper technologies, your literacy rate will zoom up and start matching the amount of beakers you have. This happens a lot when I play, as I like beelining to techs like radio and satellites often while ignoring the other techs in the previous era.