querying with index on jsonb slower than standard column. Why?

I was doing some performance profiling regarding querying against jsonb
columns and found something I can't explain.
I created json version and standard column versions of some data, and
indexed the json 'fields' and the normal columns and executed equivalent
queries against both.
I find that the json version is quite a bit (approx 3x) slower which I
can't explain as both should (and are according to plans are) working
against what I would expect are equivalent indexes.

Can anyone explain this?

Example code is here:

create table json_test (
id SERIAL,
assay1_ic50 FLOAT,
assay2_ic50 FLOAT,
data JSONB
);

DO
$do$
DECLARE
val1 FLOAT;
val2 FLOAT;
BEGIN
for i in 1..10000000 LOOP
val1 = random() * 100;
val2 = random() * 100;
INSERT INTO json_test (assay1_ic50, assay2_ic50, data) VALUES
(val1, val2, ('{"assay1_ic50": ' || val1 || ', "assay2_ic50": ' ||
val2 || ', "mod": "="}')::jsonb);
end LOOP;
END
$do$

create index idx_data_json_assay1_ic50 on json_test (((data ->>
'assay1_ic50')::float));
create index idx_data_json_assay2_ic50 on json_test (((data ->>
'assay2_ic50')::float));

create index idx_data_col_assay1_ic50 on json_test (assay1_ic50);
create index idx_data_col_assay2_ic50 on json_test (assay2_ic50);

select count(*) from json_test;
select * from json_test limit 10;

select count(*) from json_test where (data->>'assay1_ic50')::float > 90
and (data->>'assay2_ic50')::float < 10;
select count(*) from json_test where assay1_ic50 > 90 and assay2_ic50 < 10;

Thanks
Tim