Effect of caching hash bucket size while costing

Dear PostgreSQL Hackers,

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
OR
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