The subsidy is based on the second lowest cost silver plan that's available to you (this is what we mean by "benchmark"). At your income level, the subsidy will be enough so that specific plan will cost you about $5/mo. Once that amount is calculated, you can use it on any plan you want and you will just pay the difference.
What's making your subsidy lower than San Mateo's is that you have two cheap insurance options available to you in Santa Clara while San Mateo only has one. In Santa Clara, Valley Health is the lowest cost silver plan and Kaiser is the second lowest so that's the one your subsidy is based on. But Valley Health doesn't do business in San Mateo County, so there Kaiser is the lowest cost plan and Blue Shield is the second lowest. That Blue Shield plan in San Mateo is over $300 more expensive than the Kaiser plan in Santa Clara, so the subsidy in San Mateo is also over $300 higher.
Since you are looking at an HSA eligible plan, have you figured the HSA contribution deduction into your income calculations? Your actual income should be about $27150 if you plan to contribute the max.