Effect of caching hash bucket size while costing

I am working in PostgreSQL 9.4.* optimizer module. In costsize.c file and final_cost_hashjoin() function, the innerbucketsize is either:

a) calculated using a cached copy

b)  calculated afresh using statistics captured by the following code snippet:
thisbucketsize =   estimate_hash_bucketsize(root, get_leftop(restrictinfo->clause),virtualbuckets);    

For the query I used, if I disable the caching for calculating the innerbucketsize, I get a different plan with cost change of around 1000 units.

1) Can you please let me know if innerbucketsize*innerpathrows captures the maximum bucket size?
2) why is it not calculated afresh all the time?

For reference, below is the query I am using:

explain  select i_item_id,  avg(cs_quantity) , avg(cs_list_price) ,  avg(cs_coupon_amt) ,  avg(cs_sales_price)  from catalog_sales, customer_demographics, date_dim, item, promotion where cs_sold_date_sk = d_date_sk and cs_item_sk = i_item_sk and cs_bill_cdemo_sk = cd_demo_sk and   cs_promo_sk = p_promo_sk and cd_gender = 'F' and  cd_marital_status = 'U' and  cd_education_status = 'Unknown' and  (p_channel_email = 'N' or p_channel_event = 'N') and  d_year = 2002 and i_current_price <= 100 group by i_item_id order by i_item_id

and the hashclause which was tried was (item.i_item_sk = catalog_sales.cs_item_sk).

Thanks,

Srinivas Karthik