Db service connection conecting to the wrong node

I define a db service pointing to  just one node however my sql connection doesn't alwyas go to the preferred node even though it is available. 50% goes to eraps1s1 and the other 50% goes to eraps1s2. why, shouldn't all connections go to eraps1s1 given the definition below.

oracle@aprac2a ~]$ srvctl config service -d eraps1s -s ervcp1s_sv
Service name: ervcp1s_sv
Service is enabled
Server pool: eraps1s_ervcp1s_sv
Cardinality: 1
Disconnect: false
Service role: PRIMARY
Management policy: AUTOMATIC
DTP transaction: false
AQ HA notifications: true
Failover type: SELECT
Failover method: BASIC
TAF failover retries: 180