Question about album vs individual - did not use 'except'

Hi! I dont understand how to use EXCEPT on this question. Could you help with it?

Based on my understanding, I got the total number of individual and the total number of ablum are 481 and 135, but I got a total invoice_id 614. How did it happen?

Screen Link: https://app.dataquest.io/m/191/guided-project%3A-answering-business-questions-using-sql/6/albums-vs-individual-tracks

My Code:

%%sql
with ilt as (select t.*,il.* from invoice_line il 
             inner join track t on t.track_id=il.track_id),

ailt as (select ilt.invoice_id,a.album_id,count(ilt.track_id) as number_of_track,
(case when count(ilt.track_id)<=2 then 'individual' else 'album' end) as category
from album a inner join ilt on a.album_id=ilt.album_id
group by 1,2)

select count(distinct case when category='individual' then invoice_id end) as total_indi_invoice,
count(distinct case when category='album' then invoice_id end) as total_ablum_invoice,
count(distinct invoice_id) as total_invoice,
round(cast(count(distinct case when category='individual' then invoice_id end) as float)/count(distinct invoice_id),2)
as percentage_of_indi,
round(cast(count(distinct case when category='album' then invoice_id end) as float)/count(distinct invoice_id),2) 
as percentage_of_album
from ailt

What I expected to happen:

album_purchase number_of_invoices percent
no 500 0.8143322475570033
yes 114 0.18566775244299674

What actually happened:

total_indi_invoice	total_ablum_invoice	total_invoice	percentage_of_indi	percentage_of_album
481	                  135	                614	                0.78	            0.22