[LH Lead-Foot]

Nope, Its all fine. What you are seeing is voltage drop on a high current circuit. Remember, low current is less than roughly 30 amps, so across the wire from connection to connection will show this drop of 0.300 volt DC to 0.600 volts DC. High current over 50 - 60 amp of more, you can expect to see 05.00 volt DC or 1/2 of one amp. You are test for a simple voltage reading at two locations showing the difference in supply as compared to distribution where the current is split out into many circuits. Your difference is 5 volts. Now, use and amp clamp to find the actual current being used on each circuit. If you don't have one or don't need to see the current, then take you DMM from the battery positive to the positive wire at the distribution block voltage supply. Then you will see the loss or drop in voltage.
Yes, you can install a large gauge wire, but only slightly less drop in voltage. Say you have some really good jumper cables sitting around. One with more copper wire than insulation. Connect one end to your battery, the the same end to the bolt of your distribution center. It will like installing a large gauge wire. Then is you can do this safely, take you DMM and take the same reading again. Changes are, they will be near the same.
Digital readout amp clamps have come way down in price over the last decade and are surprisingly accurate. Get on the has DC and a clamp the is not too large nor too small. It can show each circuit and tell you how many amps are going thru it. Add them up at you distribution block, then check how many amps are coming out of the alternator at the same speed. It will add up real close to actual amperage being used. If you have a friend that can hold the engines speed at say, 1,500 RPM's, then measure at the alternator again.
Remember, some current is used to charge the battery depending on it's condition and how long it's been since you started it. Give it a chance to charge up with the engine running, then take you tests.