sql plan question

Any body can tell why one line change made the Card and Bytes read so much different?

before-------------------------------------------------------------------------

select MAPONL_CPTK_MC as a
,ONLOCC_ATTRIBUTABLE_ONLWS_SEQ as  b
,ONLOCC_ATTRIBUTABLE_PAGE_DEPTH as  f
,MAPONL_ONLF_FORMAT_CODE, trunc(ONLOCC_ONLSES_BEGIN_DTM) as d
,count(MAPONL_CPTK_MC) as  g
from FONL_AD_OCCURRENCES, FONL_CPTK_MAP
where nvl(ONLOCC_ATTRIBUTABLE_ONLWS_SEQ,0) > 0
and ONLOCC_ATTRIBUTABLE_PAGE_DEPTH is not null
and ONLOCC_MAPONL_SEQ=MAPONL_SEQ
and exists (select 1 from temp_three_NND where num1 = ONLOCC_ONLSES_SEQ)
and MAPONL_CPTK_MC is not null and exists (select 1 from FMONLADS_new where MONL_ADCOD = MAPONL_CPTK_MC)