The time to steal a tech is calculated like this:
t = (c*m) / b
t - turns to steal
c - 125% of the cost of the most expensive tech that you can steal, at the turn when your spy starts stealing
b - beakers per turn of the target city, at the turn your spy starts stealing
m - multipliers... constabulary, police station, autocracy policy that doubles steal rate. The exact way the multipliers work (additive or multiplicative) doesn't matter for the original question, and I can't say for certain anyway
Note that both "c" and "b" are NOT updated as time goes by, they are determined on the turn the spy starts stealing.
So, the most likely answer to the original question is that you probably did not have expensive techs that the AI could steal (or maybe they didn't have the prerequisites for those expensive techs), while the AI did have expensive techs that you could steal, driving up the value of "c".
Of course, "b" could've played a role too. It is not at all uncommon for the AI to build the national college somewhere other than their capital. You should check when the spy arrives. They also like focusing production 100% when building wonders, no joke, they will have unemployed citizens for +1 hammer instead of working an amazing tile if it doesn't have any hammers. Sometimes, depending on how long it would take to steal, it's worth it to move the spy out and back in after they finish their wonder if there aren't any other good cities. Especially if the AI in question has the NC, observatory, and rationalism policy that gives +2 science per specialist. When they finish their wonder, the specialists go back to work, and science rate can easily double.
Another important point to make is that you don't always want to steal from the runner up in tech. If for example he has some expensive tech that you can steal from the top of the tree, but you would rather steal chemistry or fertilizer, it might be a good idea to steal from someone that's a bit further behind and doesn't have that tech that you don't want that's driving up "c". It's incredibly situational, but still worth knowing.