Incremental terrain height increase limited by float size

I'm setting up a sandbox game for users where they can raise and lower terrain by clicking on the Cardboard trigger (it's a VR app).

Here's a video of how the interface works (includes console tracking): https://www.youtube.com/watch?v=UGgsdZSSBjQ&feature=youtu.be

The issue is that the user clicks a couple of times and then the heightening stops.

This, I think is due to the fact that the height reaches 0.99f (so cannot go over 1f). You can see that if you watch the console part of the video link.

Here is my code:

public class terrainEdit : MonoBehaviour { private TerrainData tData; private Terrain terrain;

 private int xRes;
 private int yRes;

 // Use this for initialization
 void Start () {
     terrain = gameObject.GetComponent <Terrain> ();
     tData = terrain.terrainData;
     xRes = tData.heightmapWidth;
     yRes = tData.heightmapHeight;

     Cardboard.SDK.OnTrigger += editTerrain;
 
 }
 
 // Update is called once per frame
 void Update () {
 
 }

 void editTerrain(){
     float currentHeight = tData.GetHeight (5, 5);
     Debug.Log (currentHeight + "before change");
     float adjustedHeight = Mathf.Clamp(currentHeight, -1.0f, 1.0f);
     Debug.Log (adjustedHeight + " after clamp and before change");
     currentHeight = currentHeight/100 + 0.01f;
     Debug.Log (currentHeight);
     float[,] heights = new float[1,1] {{currentHeight} };


     tData.SetHeights(5,5, heights);
     
 }

 private void OnDestroy(){
     this.terrain.terrainData.SetHeights (5, 5, new[,]{ { 0.0f} });
 }

}

Note that for this experiment I'm focusing on changing the height of a single point at heightmap[5,5] - which explains those hard-coded values.

I don't understand why the values are jumping - I checked against other code samples that are trying to do terrain editing at runtime and they are using the same strategy.

This must be something to do with how I manage floats - but I'm not sure what it is.