Why different prices for the same education plan?

The Educational Plan is approx 60% off the regular ALL APPS plan for the first year only.  After that, the price goes up.  In order to get the Educational Plan, you must first cancel your current Photography Plan and early cancellation fees may apply.

The Educational Discount is for the full Creative Cloud ALL APPS plan only (31+ apps & services).

https://www.adobe.com/creativecloud/plans.html

There is no such thing as an Educational discount on the Photography Plan because it is already a deeply discounted bundle. 

Creative Cloud Photography Plan -- costs USD $10/month for 12 months and includes these great products & services:
-- Photoshop CC and Photoshop on iPad
-- Lightroom on desktop, mobile and the web
-- Lightroom Classic
-- Portfolio website + hosting
-- Spark with premium features
-- Bridge
-- 20 GB cloud storage (upgradable to 1 TB).
https://www.adobe.com/creativecloud/photography.html

it's hard to say what you specifically saw, but in general, yes.  adobe offers the lowest prices to first time subscribers.