If you want to set an item for 100%, you should use X category. Another item 100%, another category. If you put few items under one category, the 100% should be divided, for example
<category id="1">
<drop itemid="45" min="1" max="1" chance="1000000"/> // 100%
</category>
<category id="2">
<drop itemid="1867" min="1" max="1" chance="100000"/> //10%
<drop itemid="1872" min="1" max="1" chance="300000"/> //30%
<drop itemid="1875" min="1" max="1" chance="550000"/> //55%
<drop itemid="1881" min="1" max="1" chance="50000"/> //5%
</category>
So, category 1 drop rate is 100% for that item, however for category 2 you have following chances (10 + 30 + 55 + 5 = 100), so there are various chances to drop random item (obviously, higher %, higher possibility for that item to be dropped).