MongoDB chunk split and jumbo chunks - mongodb

Question

Using a MongoDB default setup with max chunk size of 64Mb and max document size of 16Mb (https://docs.mongodb.org/manual/reference/limits/).
I had the issue of a chunk to big that needed to be split but the split failed.
I don't understand why. Let's assume we have big documents, if a chunk is created with a document (16Mb), then a new document comes (32Mb), and so on, until we reach 64Mb, then the split process occurs. Why can it fail? why don't split the chunk in two halves of 2 16Mb documents?
Or in the case the documents are like:
10Mb + 16Mb + 16Mb + 16Mb + 16Mb = 74Mb (needs to split)
It can do:
10Mb + 16Mb + 16Mb = 42Mb
16Mb + 16Mb = 32Mb
Why is this failing?
MongoDB log:
2016-04-06T12:10:53.424-0700 I SHARDING [Balancer] ns: reports.storePoints going to move { id: "reports.storePoints-storeId-9150629136201875191", ns: "reports.storePoints", min: { storeId: -9150629136201875191 }, max: { storeId: -9148794391694793543 }, version: Timestamp 666000|1, versionEpoch: ObjectId('54ac908475d02f0bdb362171'), lastmod: Timestamp 666000|1, lastmodEpoch: ObjectId('54ac908475d02f0bdb362171'), shard: "rs1" } from: rs1 to: rs2 tag []
2016-04-06T12:10:53.477-0700 I SHARDING [Balancer] moving chunk ns: reports.storePoints moving ( ns: reports.storePoints, shard: rs1:rs1/host1:27017,host2:27017, lastmod: 666|1||000000000000000000000000, min: { storeId: -9150629136201875191 }, max: { storeId: -9148794391694793543 }) rs1:rs1/host1:27017,host1:27017 -> rs2:rs2/host2:27017,host2:27017
2016-04-06T12:10:54.052-0700 I SHARDING [Balancer] moveChunk result: { chunkTooBig: true, estimatedChunkSize: 111379744, ok: 0.0, errmsg: "chunk too big to move", $gleStats: { lastOpTime: Timestamp 1455673163000|2, electionId: ObjectId('55b838ecf80a354067abdbbf') } }
2016-04-06T12:10:54.053-0700 I SHARDING [Balancer] balancer move failed: { chunkTooBig: true, estimatedChunkSize: 111379744, ok: 0.0, errmsg: "chunk too big to move", $gleStats: { lastOpTime: Timestamp 1455673163000|2, electionId: ObjectId('55b838ecf80a354067abdbbf') } } from: rs1 to: rs2 chunk: min: { storeId: -9150629136201875191 } max: { storeId: -9148794391694793543 }
2016-04-06T12:10:54.053-0700 I SHARDING [Balancer] performing a split because migrate failed for size reasons
2016-04-06T12:10:54.063-0700 I SHARDING [Balancer] split results: CannotSplit chunk not full enough to trigger auto-split
2016-04-06T12:10:54.063-0700 I SHARDING [Balancer] marking chunk as jumbo: ns: reports.storePoints, shard: rs1:rs1/host1:27017,host2:27017, lastmod: 666|1||000000000000000000000000, min: { storeId: -9150629136201875191 }, max: { storeId: -9148794391694793543 }