Why does the acc_trans table have fewer lines after upgrading to 1.7?

Short answer: Due to differences in the database schema in relation to storing foreign currency transactions.

Long answer: The database schema before 1.7 required 2 database rows in the acc_trans table to store functional currency and foreign currency amounts. As of 1.7, the schema stores these two amounts - which relate to the same journal line - into one database row. When migrating data from the old schema to the new one, the migration routine tries hard to identify pairs of rows which can be combined into a single row in the new schema.