mongodb $avg aggregation calculation out by a few decimals. - mongodb

Question

We have a collection in Mongodb which saves a value linked to a timestamp.
Our document looks as follows (I have pasted an actual one here):
{
"_id" : ObjectId("5a99596b0155fe271cfcf41d"),
"Timestamp" : ISODate("2018-03-02T16:00:00.000Z"),
"TagID" : ObjectId("59f8609eefbb4102f4c249e3"),
"Value" : 71.3,
"FileReferenceID" : ObjectId("000000000000000000000000"),
"WasValueInterpolated" : 0
}
What we then do is calculate the avg between two intervals for a given period, in more basic terms, work out an aggregated profile.
The aggregation code we use is:
{[{ "$match" :
{
"TagID" : ObjectId("59f8609eefbb4102f4c249e3") }
},
{
"$match" : { "Timestamp" : { "$gte" : ISODate("2018-03-12T00:00:00.001Z") } }
},
{
"$match" : { "Timestamp" : { "$lte" : ISODate("2018-03-13T00:00:00.001Z") } }
},
{
"$group" :
{
"_id" : { "GroupedMillisecond" :
{
"$let" :
{
"vars" :
{ "newMillisecondField" :
{
"$subtract" : ["$Timestamp", ISODate("2018-03-12T00:00:00.001Z")]
}
},
"in" : { "$subtract" : ["$$newMillisecondField", { "$mod" : ["$$newMillisecondField", NumberLong(1800000)] }] }
}
} }, "AverageValue" : { "$avg" : "$Value" }
}
}, { "$sort" : { "_id.GroupedMillisecond" : 1 } }
]}
The problem is this, the value it should give back is 71.3, but we get back 71.299999999999997
In this case, I posted above we are calculating the avg value, half hourly aggregated, for a day. And there is only one value per half hour logged (I checked this in the database). The value is also logged as a constant, as far back as I manually checked (a few months back) it is 71.3
So my question is why does the value differ?