Question about indexes.

Postgres Version : 6.5 (beta)
I have a table with 53411 rows.
This table has no indexes at all. Each query made (using EXPLAIN) reveals a
"Seq Scan".
So far, everything is OK.
Then I define 2 indexes, one affects a varchar field, for example 'lastname'
an the other a float type field, let´s call it 'id'.
When I perform a query such as SELECT * FROM table WHERE lastname =
'Douglas', EXPLAIN reveals that index is being used ("Index Scan").
But when the query is like: SELECT * FROM table WHERE id = 10, no index is
used, a "Seq Scan" is made. So it looks like if an index defined on a
numeric type field, doesn't work. (same thing happens with an index on
integer field).
Finally I made the index on 'id' field CLUSTERED. Repeating the query using
EXPLAIN I get a lower cost and "Index Scan".

The same test was made after deleting all records from the table and adding
a few ones (for example 100). This time a "non-clustered" index defined on
'id' field worked fine.

Is there an explanation of this behaviour?
Thanks in advance.

Spolar Alejandro.