Bug or feature? rhtml file is not using the overwritten accessor of ActiveRecord class

I was wondering about a quite strange behaviour in RoR:

I overwrote the default accessors for the "duration" field of my
ActiveRecord class "Task" to do some conversion.
In my View in the rhtml file, I tried to use:
<%= text_field 'task', 'duration' %>

When writing the duration, it correctly uses my duration= method, but
when reading the duration for displaying it in the textfield, it
somehow bypasses my overwritten duration method.

However, when i use the following code, it works for writing _and_
<%= text_field_tag 'task[duration]', @blocking.duration %>

That behaviour seems quite strange to me, does anyone have an