When an item has more than one vendor the one at the top is the default. This doesn't prevent you purchasing from other vendors but the default vendor's price will be used when calculating the value of your inventory or build cost of assemblies etc.
In our above example you see the item is supplied in a box and when you purchase this item miniMRP will process the purchase order as a quantity of boxes - an order quantity of 5 would mean 5 boxes - and when you take delivery miniMRP now knows how many are contained in each box so would automatically increase your on-hand quantity by 5000.
When processing purchase orders you can of course make any changes you wish to the quantities, prices and units of measure but such changes would only affect that specific purchase order. If you want to make permanent changes you need to do that via the Vendors tab described above.
You don't see it in the images above but when you make any changes a save button appears prompting you to save your changes.
Note: miniMrp Enterprise Edition also shows a "Currency" column. That would be used to indicate the supplier's default currency and that the price shown is in that currency. When you place purchase orders on that supplier then the currency name would appear on the order and the order is assumed to be in the supplier's own currency.